What was found
IAH- DURING FLIGHT THE CREW NOTED SMOKE IN THE CABIN AND RETURNED TO IAH WITHOUT INCIDENT. IAH- MAINTENANCE INSPECTED THE AIRCRAFT AND DETERMINED THE APU TO BE THE CAUSE OF SMOKE IN THE CABIN AND PLACED THE APU ON MEL. IAH ON 06/02/2001 MAINTENANCE REMOVED AND REPLACED THE APU, PERFORMED A OPERATIONAL CHECK AND A LEAK CHECK WITH NO DISCREPANCIES NOTED, CLEARED THE MEL AND THE AIRCRAFT WAS APPROVED FOR RETURN TO SERVICE.