This paper presents a descriptive narrative of a mathematical manpower model and the results of an analysis of the effect physician extenders have on medical costs and manpower requirements. The model is extensively developed, through the use of a new medical classification system in the area of delegation of specific task areas and patient visits to physician extenders. Additionally the models incorporate a complete cost structure for a group practice. Field trials in seven HMOs indicate that the models accurately represent the actual system and can be used effectively as planning aids. Results are presented that analyze the use of physician extenders from the following viewpoints: minimum cost solution for adult medicine, pediatrics and obstetrics/gynecology (OB/GYN); maximum physician extender use; effect of physician extender salary on minimum cost utilization; level of independence exercised; size of clinic and regional manpower planning; and a case study of HMO planning. The type of results presented include cost analysis, manpower analysis, and the types of patient visits best delegated to physician extenders (PE).