Add CommCtrl string in uses chapter and use TV_ITEM structure for setting attributes of TreeView items.
uses CommCtrl; ... procedure TForm1.TreeView1GetSelectedIndex(Sender: TObject; Node: TTreeNode); var Item: TTVItem; begin FillChar(Item, Sizeof(Item), 0); Item.hItem:=Node.ItemId; Item.Mask:=TVIF_STATE; Item.StateMask:=TVIS_BOLD or TVIS_CUT; Item.State:=TVIS_BOLD; TreeView_SetItem(Node.Handle, Item); end;