Background: In Australia, notification rates for chlamydia have increased fourfold since the early 1990s. An increasing incidence of genital Chlamydia trachomatis infection would be expected to lead to a rise in the incidence of chlamydia related diseases. Objectives: To determine trends in hospital admission rates for pelvic inflammatory disease (PID), ectopic pregnancy, and epididymo-orchitis in New South Wales. Methods: Age specific admission rates from 1992 to 2001 were ascertained using inpatient data from all hospitals within the state. Results: Among women aged 15-34 years, hospital admission rates for PID fell from 165 per 100 000 population in 1992 to 64 per 100 000 population in 2001 (p for trend<0.0001). 15% of PID cases in women aged 15-24 years were chronic, compared with 32% in those aged 25-34 years, and 46% in the 35-44 year age group. The incidence of ectopic pregnancy and admission rates for epididymo-orchitis remained constant. Conclusions: On a population level, trends in the incidence of chlamydia related diseases do not necessarily parallel those of reported chlamydia rates and ecological associations between the two need to be interpreted with caution.