30,000 individuals living in Aarhus County, Denmark by Oct 1997 were randomized into two groups. The intervention group received an invitation to be tested for urogenital Chlamydia trachomatis by use of home-obtained and mailed sample (9,000 individuals). The control group received no intervention (21,000 individuals). Outcome measures: Number of tested individuals, number of detected infections, number of women developing PID, ectopic pregnancy or infertility, number of women giving birth to a child, number of women receiving IVF treatment and number of men developing epididymitis.