Forty-one DSM-III-R hypochondriacs were obtained by screening consecutive visitors to a general medical clinic and compared with 76 patients randomly chosen from the same setting. They completed a research battery consisting of a structured diagnostic interview (DIS) and self-report questionnaires to measure hypochondriacal symptoms and functional impairment. The criterion standard for diagnosing hypochondriasis was a structured interview of demonstrated reliability and validity. The six-month prevalence of DSM-III-R hypochondriasis was estimated to be between 4.2% and 6.3% of consecutive attenders who met the inclusion criterion of having visited the same physician before. This rate was somewhat lower than that found for generalized anxiety disorder and comparable to that of alcohol abuse. The hypochondriacal patients did not differ significantly from the comparison random sample in sociodemographic risk factors except that they were significantly more likely to be Black. Hypochondriacal symptomatology was similar in males and females; and in those over 65 and those under 65. Over a three-week interval, hypochondriacal symptoms remained stable, and hypochondriacal patients had significantly higher levels of long-term disability than did the comparison patients.