Giardiasis is a common gastrointestinal illness among travelers. Recently an increased prevalence of giardiasis in men who had not traveled outside New York City was seen at The New York Hospital and was found to be due to transmission of this disease among homosexuals. Cases of giardiasis for a 5-year period were then reviewed, and it was discovered that 19 male patients who had not traveled or had an immunodeficiency disease were homosexuals. This accounted for 22% of the adult men with giardiasis during that period. Adult women with giardiasis usually were either travelers or had an immunodeficiency disease (96%). It is important to obtain a sexual history in these patients and treat sexual contacts to prevent recurrent infection. Our findings are consistent with venereal transmission of giardiasis.