Hospital infection control in Thailand was initiated in 1971, but it was not until 1987 that active infection control activities actually started. To evaluate the efficacy of the infection control programme, two national prevalence studies of hospital acquired infection (HAI) were undertaken. The HAI prevalence rate in 1988 was 11.7%; this was reduced to 7.3% four years later. The reduction of HAI was found in hospitals of all sizes, in all types of infection and almost all services. This reduction happened despite a shortage of infection control personnel. Co-operation of administrators, doctors and nurses is essential for success in HAI control. Such co-operation has been successfully created by the Nosocomial Infection Control Group of Thailand.