OBJECTIVE--To investigate asymptomatic carriage of Haemophilus ducreyi by using polymerase chain reaction (PCR) on samples from women at high risk of infection. SUBJECTS--213 commercial sex workers (CSWs) recruited in The Gambia, West Africa. METHODS--Genital samples (cervical, vaginal and ulcer) were tested for the presence of H ducreyi by PCR with the technique of "one tube nested primer". RESULTS--12 CSWs were PCR positive for H ducreyi; 8 of these women had genital ulcers on examination. CONCLUSION--Using a simplified PCR technique for detecting H ducreyi we have shown that 2% of CSWs were carrying the organism without clinical symptoms or signs. This has important implications for sexually transmitted disease control programmes in areas with a high prevalence of chancroid.