Use the NextVisibleItem property to access the visible items. The NextVisibleItem property retrieves 0 if there are no more visible items.  Use the IsItemVisible property to check whether an item fits the control's client area. Use the FirstVisibleItem property to retrieve the first visible item.

The following VB sample enumerates all visible items:

Private Sub VisItems(ByVal c As EXTREELibCtl.Tree)
    Dim h As HITEM
    With c.Items
        h = .FirstVisibleItem
        While Not (h = 0)
            Debug.Print .CellCaption(h, 0)
            h = .NextVisibleItem(h)
        Wend
    End With
End Sub

The following C++ sample enumerates all visible items:

#include "Items.h"
CItems items = m_tree.GetItems();
long hItem = items.GetFirstVisibleItem();
while ( hItem )
{
	OutputDebugString( V2S( &items.GetCellCaption( COleVariant( hItem ), COleVariant( long(0) ) ) ) );
	hItem = items.GetNextVisibleItem( hItem );
}

The following C# sample enumerates all visible items:

EXTREELib.Items items = axTree1.Items;
int hItem = items.FirstVisibleItem;
while ( hItem != 0 )
{
	object strCaption = items.get_CellCaption(hItem, 0);
	System.Diagnostics.Debug.WriteLine( strCaption != null ? strCaption.ToString() : "" );
	hItem = items.get_NextVisibleItem(hItem);
}

The following VB.NET sample enumerates all visible items:

With AxTree1.Items
    Dim hItem As Integer
    hItem = .FirstVisibleItem
    While Not (hItem = 0)
        Debug.Print(.CellCaption(hItem, 0))
        hItem = .NextVisibleItem(hItem)
    End While
End With

The following VFP sample enumerates all visible items:

with thisform.Tree1.Items
	.DefaultItem = .FirstVisibleItem
	do while ( .DefaultItem <> 0 ) 
		wait window .CellCaption( 0, 0 )
		.DefaultItem = .NextVisibleItem( 0 )	
	enddo
endwith