The prevalence of Chlamydia trachomatis was investigated in a retrospective study of 17,215 women who requested termination of pregnancy at Ullevål and Aker hospitals in Oslo during the years 1985-89 and 1991. The overall prevalence of C trachomatis decreased from 11.3% (267/2,354) in 1985 to 3.8% (112/2,958) in 1991. The reduction was considerable from 1985 to 1989 but only slight at the end of the period. The prevalence was higher in younger women during the whole period. In women or = 25 years old from 5.9% in 1985 to 1.8% in 1991.