During a study of genital infection in inner-city family-planning patients we examined 452 women for Chlamydia trachomatis. The prevalence of infection was 7·3%. There was no significant difference between patients attending because of genital symptoms and those who were attending for routine family-planning advice. Infection was found to be correlated with five main demographic parameters; age less than 25, no stable partnership, hormonal contraception, nulliparity and West Indian Ethnic origin. Using these parameters a simple scoring system was devised which allowed a high-risk population to be defined in whom screening would be economically justified.