In the United States statistics on sexually transmitted diseases (STDs), other than gonorrhoea and syphilis, are meagre. In this study the relative and seasonal incidences of most STDs in an American clinic where 34,938 patient visits were recorded over a two-year period (1975-76) are assessed. Gonorrhoea was the most common STD in male and female patients combined (18%), while nongonococcal urethritis (NGU) was most common in men (23%), and vaginitis (trichomonal 7.5%, yeast 7.1%, and non-specific 7.1%) was the most common in women. A significantly higher incidence of NGU occurred in Caucasian (63%) than in black (42%) men (P less than 0.005). No other STD was diagnosed in more than 5% of patients, and 31% had normal findings on clinical examination and investigation, and could be described as the 'worried well'. Two or more STDs co-existed in 4.2% of patients. In 1976 the incidence of genital herpes and scabies decreased in contrast to other STDs and total patient visits, which increased. A seasonal peak in late summer and early autumn was observed for most STDs. These observations indicate the importance of a comprehensive approach when attempting to compile accurate statistics on selected epidemiological aspects of sexually transmitted diseases.