In 1979-80, 21,329 subjects, 20-54 years old, were invited to a screening for coronary heart disease risk factors in Tromsø. 16,621 (77%) attended the screening, the response rate was higher in women than in men, lower in single than married and lowest in young (20-29 years) men. At the screening, the men and women were given a second questionnaire which they were asked to fill in and return by mail. 14,667 (88.2%) of the subjects did so. Based on information obtained at the screening, the differences between these 14,667 subjects and the 1954 men and women who failed to return the questionnaire are presented. 10.9% of the women did not return the questionnaire, the corresponding figure for men was 12.6%. The differences in age, body mass index (kg/m2), blood lipids and blood pressure were minor or non-existent. The subjects who returned the questionnaire tended to be married, non-smokers and report respiratory symptoms less often than non-responders.