Human immunodeficiency virus (HIV) infection and AIDS is threatening the survival of many nations. To evaluate ongoing interventional strategies and burden of illness estimates, valid data on the prevalence of HIV are required. Often, in the absence of community prevalence data, estimates are based on surrogate markers such as prevalence of HIV in antenatal clinics. Even though the antenatal prevalence of HIV is easier to measure and can be repeated for evaluation, it is important to establish the association between antenatal and community prevalence of sexually transmitted diseases (STDs) and HIV, so that the validity of the estimates can be verified. A 'probability proportional to size' cluster survey was conducted in three randomly selected districts of Tamil Nadu in India. The basic unit of the survey was households from rural and urban clusters. Adults 15-45 years of age from the selected households were eligible for recruitment. Demographic, behavioural and laboratory data were collected. Clinical examination was done to identify STD syndromes and blood, urine, vaginal/urethral and endocervical swabs were taken for laboratory diagnosis of STDs from the subjects. Direct smear examination for Trichomonas vaginalis; serological tests for syphilis, hepatitis B, HIV, herpes simplex virus 2, Chlamydia trachomatis; and culture of Neisseria gonorrhoeae and Haemophilus ducreyi were performed on the collected specimens. The data were analysed adjusting for cluster effect. We selected and screened 1981 individuals (1157 women and 824 men) for STDs and HIV from 1114 households representing the 25 million projected adult population of Tamil Nadu. The overall community prevalence of STDs including HIV and hepatitis B in Tamil Nadu was 14.6% (CI: 14.1-15.1), and 8.3% (CI: 7.9-8.6) when HIV and hepatitis B were excluded. Community prevalence of HIV and hepatitis B infection was 1.8% (CI:1.7-1.9) and 5.3% (CI: 5.1-5.5), respectively. The distribution of HIV involved both rural and urban regions of Tamil Nadu. On clinical examination, at least one STD syndrome was noted in 486 (24.5%) of the women subjects; vaginal discharge was the most common and found in 421 women (38.4%). The prevalence of STD and HIV in Tamil Nadu is higher than expected and has extended into the non-high risk population (generalized epidemic).