Background: Sexually transmitted infections (STIs) present a huge burden of disease and adversely affect the reproductive health of people. The disease prevalence is about 6% in India. This study is done to determine the prevalence of STIs in women of reproductive age (15-49yrs) attending gynaec outpatient block at Institute of Obstetrics and Gynaecology, Chennai and to identify the risk factors.Methods: It is a prospective analytical study conducted at IOG from Febraury 2010 to January 2011 where 1000 women of reproductive age attending gynec op were included of which 500 women were asymptomatic and 500 women were symptomatic for STIs. A well-structured proforma was prepared for selection of women, history, examination, investigations and treatment. Asymptomatic women were also screened and treated. Partners were also screened and treated. Depending upon the statistical data, the risk factors were identified and analysed.Results: Overall prevalence of STI was 27.2% of which 22.5% was in symptomatic group and 4.7% in asymptomatic group. Bacterial vaginosis was the commonest STI. The important risk factors identified were age group between 26-30 years, high risk sexual behaviours, poor socioeconomic factors, poor menstrual hygiene and lack of contraception.Conclusions: STIs cause major health problem and it is important to diagnose and treat them at the earliest. The importance of STIs has been more widely recognised since the advent of the HIV/AIDS epidemic, and there is good evidence that their control can reduce HIV transmission. Women diagnosed with one STI should be screened for other STI due to coexistant infections. Screening and treatment of partners and follow-up tests of cure should be performed wherever possible.