Index: src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFRow.java
===================================================================
--- src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFRow.java	(revision 1510967)
+++ src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFRow.java	(working copy)
@@ -40,7 +40,9 @@
     short _height=-1;
     boolean _zHeight = false;
     int _outlineLevel = 0;   // Outlining level of the row, when outlining is on
-
+    Boolean _hidden;
+	Boolean _collapsed;
+	
     public SXSSFRow(SXSSFSheet sheet, int initialSize)
     {
         _sheet=sheet;
@@ -61,7 +63,22 @@
     void setOutlineLevel(int level){
         _outlineLevel = level;
     }
+    
+    public Boolean getHidden() {
+		return _hidden;
+	}
+
+	public void setHidden(Boolean hidden) {
+		this._hidden = hidden;
+	}
 
+	public Boolean getCollapsed() {
+		return _collapsed;
+	}
+
+	public void setCollapsed(Boolean collapsed) {
+		this._collapsed = collapsed;
+	}
 //begin of interface implementation
     public Iterator<Cell> iterator()
     {
Index: src/ooxml/java/org/apache/poi/xssf/streaming/SheetDataWriter.java
===================================================================
--- src/ooxml/java/org/apache/poi/xssf/streaming/SheetDataWriter.java	(revision 1510481)
+++ src/ooxml/java/org/apache/poi/xssf/streaming/SheetDataWriter.java	(working copy)
@@ -142,6 +142,13 @@
         if (row.getOutlineLevel() != 0) {
             _out.write(" outlineLevel=\"" + row.getOutlineLevel() + "\"");
         }
+        if(row.getHidden() != null) {
+        	_out.write(" hidden=\"" + (row.getHidden() ? "1" : "0") + "\"");
+        }
+        if(row.getCollapsed() != null) {
+        	_out.write(" collapsed=\"" + (row.getCollapsed() ? "1" : "0") + "\"");
+        }
+        
         _out.write(">\n");
         this._rownum = rownum;
         _rowContainedNullCells = false;
Index: src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFSheet.java
===================================================================
--- src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFSheet.java	(revision 1510481)
+++ src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFSheet.java	(working copy)
@@ -1115,10 +1115,61 @@
      */
     public void setRowGroupCollapsed(int row, boolean collapse)
     {
-        //_sh.setRowGroupCollapsed(row, collapse);
-        throw new RuntimeException("Not Implemented");
+    	if (collapse) {
+            collapseRow(row);
+        } else {
+            //expandRow(rowIndex);
+        	throw new RuntimeException("Not Implemented");
+        }
     }
+    
+    /**
+     * @param rowIndex the zero based row index to collapse
+     */
+    private void collapseRow(int rowIndex) {
+        SXSSFRow row = (SXSSFRow) getRow(rowIndex);
+        if (row != null) {
+            int startRow = findStartOfRowOutlineGroup(rowIndex);
 
+            // Hide all the columns until the end of the group
+            int lastRow = writeHidden(row, startRow, true);
+            SXSSFRow lastRowObj = (SXSSFRow) getRow(lastRow); 
+            if (lastRowObj != null) {
+            	lastRowObj.setCollapsed(true);
+            } else {
+                SXSSFRow newRow = (SXSSFRow) createRow(lastRow);
+                newRow.setCollapsed(true);
+            }
+        }
+    }
+	
+    /**
+     * @param rowIndex the zero based row index to find from
+     */
+	private int findStartOfRowOutlineGroup(int rowIndex) {
+        // Find the start of the group.
+        int level = ((SXSSFRow) getRow(rowIndex)).getOutlineLevel();
+        int currentRow = rowIndex;
+        while (getRow(currentRow) != null) {
+            if (((SXSSFRow) getRow(currentRow)).getOutlineLevel() < level)
+                return currentRow + 1;
+            currentRow--;
+        }
+        return currentRow;
+    }
+	
+	private int writeHidden(SXSSFRow xRow, int rowIndex, boolean hidden) {
+		int level = xRow.getOutlineLevel();
+		SXSSFRow currRow = (SXSSFRow) getRow(rowIndex);
+
+		while (currRow != null && currRow.getOutlineLevel() >= level) {
+			currRow.setHidden(hidden);
+			rowIndex++;
+			currRow = (SXSSFRow) getRow(rowIndex);
+		}
+		return rowIndex;
+	}
+
     /**
      * Sets the default column style for a given column.  POI will only apply this style to new cells added to the sheet.
      *