This study sought to determine optimal serial-to-monograph ratios for collection development by comparing citation frequency with current library practice. Internal medicine literature cited an average of 88% serial references and 12% monographs. In an observational study, teaching physicians on internal medicine rounds cited 89.5% serials and 10.5% monographs to student teams. By contrast, health sciences libraries included in the Houston statistics spend an average of 79% of acquisitions budgets for serials and 21% for monographs. An 88:12 acquisitions budget ratio would be more appropriate, reflecting actual use of serials and monographs in the health sciences.