Accessing GUI from thread

Problem: I have a windows application created with Microsoft VC++ that is implemented as a Dialog window. The application has buttons to press for different actions f.ex. an “Exit” button to be used when the application shall be closed down. The application even has a Thread that runs as background worker. The thread listens for commands from a serial port.

What i want is some way for the Thread to call
the dialog (handle to the window ??) when the “exit” command is received from the serialport.

I have tried to use the SendMessage() from the Thread but without success. Is it the right solution but implemented wrong by me or does anyone have a better solution.

/With hope of help
Bj?rn