We assessed prevalence and risk factors for human immunodeficiency virus (HIV) infection in 637 patients (506 men, 131 women; median age 30 years, range 17-64) attending between September 1988 and July 1989 for the first time two sexually transmitted disease (STD) clinics in Northern Italy, for suspected or STD treatment. A total of 44 subjects (6.9%, 95% confidence interval, (Cl): 4.9-8.9) were seropositive for HIV antibodies. The prevalence of HIV infection decreased with age, from 9% in patients aged 24 years or less to 3% in those aged 45 years or more (chi 21 trend 4.97, p less than 0.05). Women tended to have a lower prevalence of infection than men (5.3% versus 7.3%) but this was not statistically significant. Compared with men reporting no homosexual intercourse, HIV infection risk was about 50% higher in those reporting bisexual intercourse (age- and sex-adjusted odds ratio (OR) 1.5,95% Cl: 0.6-3.6) and about fourfold in those reporting only homosexual intercourse (OR 3.8, 95% Cl: 1.7-8.5). No clear trend in risk was observed with number of sexual partners both in men and in women. Intravenous drug users had an increased risk of HIV infection; compared with non-users, the OR was 5.6 (95% Cl: 3.0-10.5) in users, and the point estimates increased with frequency of use, from 3.3 (95% Cl: 0.8-11.5) in occasional users to 6.4 (95% Cl: 3.2-12.8) in regular users. The risk of HIV infection was 2.2 (95% Cl: 1.1-4.3) in patients reporting a history of STD, and 1.6 (95% Cl: 0.8-3.3) in those reporting syphilis.(ABSTRACT TRUNCATED AT 250 WORDS)