As resources for family planning programs become more scarce, measurement of program efficiency is rapidly becoming one of the top priorities of family planning program evaluators. This study of Thailand is part of the continuing effort to evaluate the efficiency and impact of program inputs on family planning achievement and fertility. Analysis is restricted to 70 of Thailand's provinces as they existed from 1975-1979. Data falls into 4 categories: 1) data on government family planning inputs, 2) 1970 census data on the environmental or socioeconomic conditions, 3) data on family planning achievement that came fron the National Family Planning Program (NFPP) service statistics report for December 1978, and 4) 1980 census fertility data. Composite variables were created to improve sensitivity, correlation matrices were run to select the most important variables, and multiple regression and path analysis were used define the effects of the main independent variables on the dependent variables. The study concludes that: 1) Thailand's NFPP has an observable effect on family planning use which, in turn, has had a strong impact on reducing fertility. 2) This impact is the result of interaction between the modernization of the environment and the availability of health personnel and their services. Either factor alone has only a moderate effect on family planning use and fertility. However, when the 2 factors increase together in the same area, there is a marked impact on fertility after several years. These findings may enable the NFPP to define optimal amount of inputs for a given socioeconomic setting, resulting in more efficient use of decreasing resources.