uses 
  CommCtrl; 

procedure SetNodeBoldState(Node: TTreeNode; Value: Boolean); 
var 
  TVItem: TTVItem; 
begin 
  if not Assigned(Node) then Exit; 
  with TVItem do 
  begin 
    mask := TVIF_STATE or TVIF_HANDLE; 
    hItem := Node.ItemId; 
    stateMask := TVIS_BOLD; 
    if Value then state := TVIS_BOLD  
    else  
      state := 0; 
    TreeView_SetItem(Node.Handle, TVItem); 
  end; 
end; 

// Example: Make the first node bold. 
// Beispiel: Erster Eintrag fett machen. 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  SetNodeBoldState(TreeView1.Items[0], True); 
end;