Residency training in New York State was substantially altered by the Libby Zion case. Work-hour limitations and augmented supervisory requirements changed the patterns of training--particularly in internal medicine--but with uncertain impacts on the quality of education and patient care. In this historical analysis, we review another major effect of the case: a substantial augmentation of the number of trainees. The need to maintain adequate inpatient staffing--within the ground rules of the Residency Review Committee, and in consideration of the reimbursement formulae and financial climate of New York State--conspired to promote substantial residency program expansion. Similar forces contributed to a national trend to increase the number of trainees. The history, cost and impact of these personnel changes are reviewed.