Background: Public health authorities want to evaluate their sexually transmitted disease (STD) surveillance systems to promote the most effective use of health resources. Goal: The goal of this study was to estimate the sensitivity of national laboratory reports of Chlamydia trachomatis in Switzerland (the proportion of cases detected by national laboratory reports). Study Design: A cross sectional prevalence study was conducted by the Swiss Sentinel Surveillance Network of Gynecologists in 1998. Two groups of women aged less than 35 years were included in the study: those having a first consultation for pregnancy and those having a routine check-up. Results: A total of 1589 women were tested for C trachomatis. The prevalence among pregnant women (n = 817) was 1.3%, and that among sexually active women (n = 772) was 2.8%. Using the prevalences observed among check-up women, we estimate that there were at least 24,400 C trachomatis infections in Switzerland among women aged 20 to 34 years in 1998 (95% CI: 14,300-34,300). The number of labo-ratory reports of C trachomatis in this age group was 1150 in 1998. Conclusion: Our study suggests that the sensitivity of national laboratory reports of C trachomatis in 1998 was less than 5% for women aged 20 to 34 years.