Utilization of physician services by initial subscribers to a Canadian consumer- sponsored medical care plan was examined for the period 1965-1979. After stratification by sex and age, 1,046 individuals aged 18-44 years were sampled and their records abstracted to yield the annual number of in-plan physician visits. The most frequently attending 10% of patients generated almost 30% of all visits. Moreover, those who initially made most frequent use of physician services persisted in doing so over the 15-year period. Utilization patterns differed between the sexes. Women, with a median of 40 visits, made more frequent use of services than men, with a median of 27 visits. Among men, the group that was oldest at enrollment utilized services more heavily than their younger counterparts, and this increased relative utilization became more pronounced with time. Corresponding effects were not seen in women, even after adjustment for prenatal visits. Adjustment for prenatal visits also had only a minimal effect on the frequency and duration of nonattendance. Ascertainment of out-of-plan utilization indicated that for the most recent year of follow-up, 20% of the patients made at least one out-of-plan visit. Although this had little effect on patients' relative utilization status, it had a substantial effect on the identification of nonattenders.