The Säkylä-Köyliö Project concerned the investigation and evaluation of different screening programmes and the possibilities for providing the necessary treatment and follow-up at the health centre level. The variables studied consisted of anaemia, bacteriuria, high blood pressure, and mental health problems; an epidemiologic screening for sight and hearing defects and diseases of the musculoskeletal system was carried out as well. The use of health services, together with the relationships between objectively and subjectively determined morbidity, was also studied. A total of 2268 inhabitants of the municipalities of Säkylä and Köyliö, i.e. 93.2% of the 40- to 64-year-old female population and 93.4% of the males in the same age range, participated in the screening programme conducted by the Säkylä-Köyliö Project between 9 September 1973 and 11 January 1974. Only one-fifth (1.9%) of the non-respondents to the first invitation (8.6%) participated in the screening programme after they received a second invitation. This result demonstrates that those who do not participate for some reason cannot, to any great extent, be influenced by a renewed invitation. Since the non-participation rate was insignificant, the material examined may, from the epidemiologic point of view, be considered as representative of the middle-aged population of the two municipalities, which together form a health centre district in Southwest Finland. The high participation percentage also shows that the prerequisite of screening, namely, a positive attitude on the part of the population, was as well fulfilled in the present project.