As health maintenance organizations (HMOs) and managed health care systems expand, they represent an increasing potential as sites for medical student teaching. Considerable difference of opinion exists about the impact of medical training on these prepaid delivery systems. This study presents a methodology for estimating the subjective and objective costs and benefits of medical student training to an independent staff model HMO with a long-standing training program. Data are derived from a provider survey, a consumer survey, and patient visit logs. Principal subjective benefits include increased perceived quality of care, improved patient satisfaction, and enhanced provider education and joy of practice. Objective impacts include a decrease in productivity of 1.1 patient visits per half day and direct physician teaching labor of 46.8 minutes per half day. Applying this methodology to the specific program of ten courses gives rise to a figure of $180 000 ($16 900 per full-time equivalent student per year) for the "opportunity cost" of medical student training to the HMO. Rules of thumb are developed for application of this method prospectively to new programs in similar relationships between staff model HMOs and academic medical centers.