Autism prevalence studies have often relied on administrative prevalence or clinical diagnosis as case-identification strategies. We report the "incidence" of "clinical diagnoses" of autism spectrum disorders (ASD), versus "research-identified" autism among residents of Olmsted County, Minnesota, age [less than or equal] 21 years, from 1976-1997. The incidence of "clinically diagnosed" ASD (with 95% CI) was 1.5 per 100,000 (0.0-3.7) in 1980-1983 and 33.1 (22.8-43.3) in 1995-1997, a 22.1-fold increase. In contrast, the incidence of "research-identified" autism increased from 5.5 (1.4-9.5) per 100,000 to 44.9 (32.9-56.9), an 8.2-fold increase. Only 46.8% of research-identified cases received a clinical diagnosis of ASD. These findings demonstrate the potential for misleading interpretation of results from epidemiologic studies that rely on clinical diagnosis of autism to identify cases.