After termination of a double-blind, randomized study on erythromycin in the prevention of post-abortion infection, 34 women (14 treated with erythromycin, 20 not treated with erythromycin) harbouring Chlamydia trachomatis were followed up within 6 weeks and again 2 to 24 months after the abortion in order to detect an early- and late-onset pelvic inflammatory disease (PID). For statistical analysis survival analysis by Kaplan-Meir estimates and Mantel-Cox test were carried out. Untreated women with C. trachomatis infection at the time of abortion had a cumulative risk of 72% of developing early and/or late PID, if observed for 24 months. This cumulative risk was significantly reduced to 8% if the C. trachomatis infection was treated at the time of the abortion. Screening for and treatment of C. trachomatis is warranted, especially in women <or = 25 years old, to avoid early and late-onset PID after induced first trimester abortion.