In case-control studies in which cases are ascertained from hospitals, controls are frequently chosen from among patients with other diseases at the same hospitals. This study was undertaken to examine the extent to which a hospital control group is representative of the population to which inferences are made. A hypothetical hospital control group was assembled consisting of 233 men and women aged 40-74 years who were surgical inpatients at the two hospitals in Otsego County, New York, in 1990. The characteristics of this group were compared with the characteristics of 15,563 men and women aged 40-74 years who participated in a privately conducted health census in the same county in 1989 with the use of health-related data collected in the census. In this rural setting, only small differences were found between the hospital control group and the census population on most of the measures considered, including demographic characteristics, certain health behaviors, and the prevalence of common conditions. However, the female hospital controls were more likely to be overweight than the females enumerated in the census, and the men were more likely to have ever smoked cigarettes. These differences were large enough to lead to different interpretations about the strength of the associations between these variables and case-control status.