The role of nitrite was evaluated between 1985 and 1988 in a study of sexual transmission of the human immunodeficiency virus (HIV) among homosexual male couples in Boston, Massachusetts. Initial enrollment data suggested that a history of unprotected receptive anal intercourse (odds ratio (OR) = 2.3, 95% confidence interval (CI) 1.4-3.6) and a history of nitrite use (OR = 1.7, 95% CI 1.1-2.5) were independent risk factors for HIV infection. In addition, interaction between nitrite use and unprotected receptive anal intercourse was observed (OR = 5.5, 95% CI 2.8-11.1) after controlling for number of unprotected receptive anal sex partners and history of sexually transmitted diseases. Since it was felt that nitrite use might be a marker for unprotected receptive anal sexual activity, a supplemental questionnaire was administered to obtain information on simultaneous nitrite use and unprotected receptive anal intercourse. The supplemental data suggested a strong interaction between nitrite use and unprotected receptive anal intercourse in increasing the risk of HIV infection. In the adjusted analyses, the odds ratio for HIV infection was considerably greater among men who always used nitrites during unprotected receptive anal intercourse (OR = 31.8, 95% CI 12.9-76.7) compared with men who sometimes (OR = 7.1, 95% CI 2.1-23.6) or never (OR = 9.0, 95% CI 2.5-32.1) used them. These findings have preventive public health implications and may add insight into our understanding of the mechanism by which HIV infection spread rapidly among homosexual men in the early 1980s.