This article estimates the effects of personal, clinical, physician, and hospital characteristics in a simultaneous equations model of length of stay and ancillary services use for five narrowly defined medical and surgical conditions. These are tonsillectomy and adenoidectomy, gastroenteritis and colitis, inguinal hernia, coronary heart disease, and cholelithiasis. The data are derived from a sample survey of medical and financial records of patients discharged from any of 63 New England short-term general hospitals during the period July 1, 1969 through June 30, 1970. The results confirm the importance of a simultaneous equations formulation of utilization analysis and of inclusion of detailed measures of severity. Length of stay and ancillary services are significantly interrelated for all five conditions, corroborating results of a previous study of obstetric cases. Results for patient's employment status and value of time, attending physician specialty and mode of practice, and hospital size, control, and nature of teaching activities were less conclusive, but suggest differential effects across diagnoses, thus emphasizing the importance of a diagnostic-specific approach to utilization analysis.