We examined the effect of a questionnaire addressing sexual behaviour on participation in a systematic screening programme for asymptomatic Chlamydia trachomatis infections. Furthermore, we compared participation among persons requested to mail a home-obtained urine sample directly to the laboratory and persons requested to bring a sample to the physician's office. Seven hundred and fifty men and women were randomly assigned to receive a questionnaire with or without intimate questions and to deliver or mail the samples. The inclusion of questions about sexual behaviour did not affect participation among both men and women. Among women there was no difference in participation between delivering or mailing the sample. Among men delivering the sample, participation was 18% (95% confidence interval [CI]: 5-32) lower. This study shows that questions on sexual behaviour can be included in a screening questionnaire without adversely affecting participation. Furthermore, mailing the specimens is the most efficient strategy for men, when screening for C. trachomatis by means of home-obtained urine specimens.