The demographic characteristics and ambulatory use of health maintenance organization (HMO) members who voluntarily disenrolled from an HMO--those who returned to the alternative health insurance plan--or who involuntarily disenrolled due to job loss or change--were compared with those of persons still enrolled. The database for all three samples was comprised of ambulatory use data for 6 months (July 1979-December 1979). The HMO-enrolled sample made significantly more visits for health services, on the average, than either of the disenrolled samples. Upon controlling for demographic variables (age, sex, family size, and years enrolled), it was ascertained that young to middle-aged women from families of three or more, making visits for chronic problems, made the greater use among the enrolled sample. The relationship of these findings to the risk vulnerability theory of HMO enrollment choice is discussed, and long-term implications for HMOs are presented.