Participation rates at the first (1985) MONICA survey (representative samples of the Cantons of Vaud, Fribourg and Tessin) are presented. Sample members were invited by phone or letter to the survey (physical examination, blood sample and questionnaire). Overall participation rate was 57% in the french-speaking and 78% in the italian-speaking region. Underrepresented were resident foreigners, women, non-married persons and the aged. If no phone contact was materialized, participation was low. An additional questionnaire for non-responders allows to estimate potential biases in the results of the first MONICA survey.