Despite the fact that incidence studies of sexually transmitted diseases have rarely been performed in rural areas, it is generally assumed that the frequency of such diseases in rural areas is lower than in cities. We studied the incidence of four sexually transmitted diseases in a rural community (population 150,000) located in southern Quebec. Between March and September 1986, a random sample of 41 of 123 general practitioners reported all cases of syphilis, gonorrhea, chlamydial genital infection, and genital herpes at the time of diagnosis. In addition, all the specialists of the area who regularly diagnose sexually transmitted diseases reported their cases. For each case contact tracing was attempted. The estimated incidence rate was 584 per 100,000 person-years for at least one of the studied diseases. For chlamydia, gonorrhea, and herpes, the rates per 100,000 person-years were 470, 149, and 36, respectively. Over 11% of the cases had more than one of these diseases. Of the 114 index cases diagnosed by physicians, 102 were symptomatic, whereas of the 62 contact cases, only 25 were symptomatic (p less than 0.001). In addition, for genital chlamydial infection 84% of female cases were primarily diagnosed by physicians, whereas 64% of male cases were found by contact tracing (p less than 0.001). This study shows that sexually transmitted diseases are a major public health problem in rural communities.