If  ComboBox1.ItemIndex >= 0 ComboBox1.Items.Lines[ComboBox1.ItemIndex] then ...

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    cmb: TComboBox;
    lbl: TLabel;
    procedure cmbChange(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.cmbChange(Sender: TObject);
begin
  if cmb.ItemIndex >= 0 then
    lbl.Caption := cmb.Items.Strings[cmb.ItemIndex];
end;

end.