BACKGROUND Large-scale vaccination programmes have been carried out for a long time in Sweden, as in many other countries. However, often little is known of the effects of these vaccinations. During 1990 and 1991 a survey of immunity based on a random adult population sample was carried out. The main purpose was to estimate the level of immunity to diphtheria, tetanus and polio of the adult population in Sweden. In total 4800 people were randomly selected according to a stratified, two-stage, sampling plan. METHODS Based on standard sampling theory, methods for calculations of estimates and confidence intervals of the proportion of the population that is immune are given. The response patterns and its possible effects on the estimates are discussed. RESULTS In total, 70.6% of the 4800 selected gave a blood sample. The response rate differs for men and women and for different age groups. Among the oldest, the response rate was close to 80%, but it was only about 60% among the youngest. CONCLUSIONS With the survey design used, it was possible to obtain a sufficient degree of response. Our experience is that the response rate depends to a large extent on the efforts made to explain and motivate participation.