In order to determine the prevalence and characteristics of sexually transmitted diseases (STDs) in pregnant women (PW) attending a primary health care antenatal clinic (ANC) in metropolitan Dar es Salaam, Tanzania, a randomly selected sample of PW in their second or third trimesters were invited to participate at their first visit. They were interviewed using a questionnaire and underwent genital examination. Genital swabs were obtained for microscopy and/or culture isolation of Candida albicans, Trichomonas vaginalis, and Neisseria gonorrhoeae. Blood specimens were also obtained for serological testing for syphilis and for antibodies to the human immunodeficiency virus (HIV). A total of 777 PW aged 14 to 40 years were seen. Parities ranged from 0 to 10. Prevalence of syphilis, trichomoniasis, gonorrhoea and HIV infection were 4.0%, 22.7%, 3.6% and 15.2%, respectively. At least one acute STD (excluding HIV infection) was found in 32.8% of the PW. The prevalence of multiple STDs (excluding HIV infection) was higher in teenagers (45.3%, 77/170) than in PW in other age groups (29.2%, 177/607) (p < 0.001). The prevalence of HIV infection in teenage PW was 10.0%. Most STDs were least prevalent in PW who were married monogamously. Of the 732 PW who had one or more genital infections (including infection with Candida species), 669 (91.4%) had one or more genital complaints. However, most of the genital complaints were not disease specific. Since this study has shown that the prevalences of acute STDs were high in PW, especially in teenagers, it is recommended that all PW in Tanzania should be screened for STDs syndromically including the use of appropriate clinical and laboratory examination whenever possible.