A cost-effectiveness study of the Thai expanded programme on immunization was carried out in district hospitals and health centres in Thailand during early 1987.  The total annual spending on immunization was US$3852 in hospitals and US$813 in health centres. The percentage distribution of annual costs was similar in both facilities. Salaries were the largest component, followed by building and vaccine costs. The frequency of immunization sessions was the most important factor in determining total costs immunized children was 184, compared with 49 in health centres. The cost per fully immunized child varied widely form US$5.3 to US$33.20, and the most cost-effective facilities were those that immunized the greatest number of children.  With the present number of health facilities in all areas of the country, which correspond to saturation levels, the most likely way for the Thai programme to reduce costs would be to make better use of staff time by decreasing the frequency of the services offered, thereby increasing the efficiency of each session. Hospitals should adjust the frequency of their immunization sessions according to the number of children begin served, but health centres should offer sessions only monthly or once every two months.