What was found
MCO - FLT 4932, ON TAKEOFF AT 1200 FT THE CREW RECEIVED A LAV SMOKE WARNING, AND THE AIRCRAFT STARTED FILLING UP WITH SMOKE. THE SMOKE COLOR WAS DARK GREY STARTED AT REAR OF A/C FILLED UP ALL THE WAY TO COCKPIT. THE AIRCRAFT RETURNED TO IAH WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE REMOVED AND REPLACED A CABIN DISTRIBUTION DUCT, PERFORMED ENG RUNS, BLEED OPS AND ICE TEST WITH NO DEFECTS NOTED, AND THE AIRCRAFT WAS APPROVED FOR RETURN TO SERVICE.