To learn the main operational, professional, and business characteristics of U.S. radiology group practices, researchers at the American College of Radiology surveyed these groups. Major findings included the following: Approximately 30% of groups provide only diagnostic radiologic services, a similar percentage provides only radiation therapy for oncologic patients, and the remainder provides both types of services. Forty-one percent of groups practice only in hospitals, 11% practice only in an office, and 48% practice in both settings. Diagnostic-only practices average 10,000-12,000 procedures per full-time equivalent radiologist per year. Groups typically require new members to be part of the group for almost 3 years before they become full partners. Formal call schedules are nearly universal among radiology groups. Groups are becoming increasingly involved with health maintenance organizations and other "alternative delivery systems," but fee-for-service remains by far the dominant source of groups' revenue. Most studied characteristics of groups are changing relatively slowly, and trends are generally toward increasing formalization of arrangements.