The calculation of primary physician and physician extender needs in a prison or jail health service is based on (1) the hours of coverage for entrance medical evaluation according to population turnover rate; (2) hours of coverage for sick call services; (3) hours of coverage for chronic care clinics, infirmary care, and convalescent care; (4) hours of coverage for physician administration, supervision, internal audit, inservice education, and emergency care; and (5) the time consumed by inefficiency factors, vacations, continuing education, etc. One table presented uses the specific measurements derived for each program component to calculate the number of primary physician FTE's required in a prison with 2,000 inmates and a turnover rate of 600 new admissions annually. In this setting, 2.2 FTE's would be required. Thus, the ratio of 1,000:1 or greater internees and primary care physician currently used to designate a correctional institution as medically underserved conforms well to the minimal primary care physician needs derived from the program component formula. The current Federal regulations, however, may overestimate primary care physician needs if the ratio of internees/primary care physician remains defined simply as the number of new admissions per year rather than average daily population. The formula presented provides for consideration of both turnover rates and average daily population. Seventeen references are provided.