The national immunization coverage in Thailand for all types of vaccine has been steadily increasing since 1978, when the EPI was formally launched. The coverage in 1987 was 96% for BCG, 75% for DPT, 74% for OPV, and 60% for TT. Measles vaccine, which started only in late 1984, had the lowest coverage, 51%, in 1987. During the period 1982-1987, the drop-out rates between the first and third dose of DPT and OPV decreased dramatically from 69% to 13% and from 42% to 13% respectively. Sampling surveys of immunization coverage showed higher coverage for DPT and OPV than those from reporting in all regions, especially in the capital city which has a high concentration of the private health sector. Only the northeastern region had less coverage from surveys than from reporting. Following the launch of EPI, the disease incidence demonstrated a clearly downward trend for diphtheria, poliomyelitis, and measles, while in the case of pertussis and neonatal tetanus, slower of still fluctuating declines were observed. The reported age-specific incidences per 100,000 population in 1986 for children 0-4 years were as follows: 4 for diphtheria, 0.9 for poliomyelities, 180 for measles, 14 for pertussis, and 10 for tetanus.