A prospective population-based surveillance system was established to characterize the epidemiology of cat-scratch disease (CSD) among residents of Connecticut who were reported to the state health department with a diagnosis of suspected CSD. During 1992 and 1993, 246 persons met the case definition, for an average statewide annual incidence of 3.7/100,000 persons. The median age of patients with CSD was 14 years (range, 1-64), and 52% were female. The age-specific attack rate was highest among persons < 10 years of age (9.3/100,000) and decreased with increasing age. Symptoms in addition to adenopathy were noted by 74% of case-patients. Eleven percent of all case-patients were hospitalized. There were no deaths. Most patients with clinically diagnosed CSD developed an immunologic response to Bartonella species. Our data suggest that although CSD is primarily a disease of younger persons, the age spectrum is wider than was commonly appreciated.