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Internal overflows occur when the nursing unit that would normally treat a patient is full and the patient must be assigned to a substitute unit. This common problem in hospital capacity planning is also known as bed borrowing. A stochastic model of the external and internal patient flows among the 20 nursing units in the adult medical division of a large university hospital was formed to estimate the frequency of internal overflows. Model parameters were estimated by tracking admissions, discharges, and transfers between units for one year. Internal overflows in the stochastic model were quite common. Cases where all related units that might reasonably provide comparable care were also full, were less common but occurred frequently enough in some nursing units to cause concern. A simplified version of the model allowed computation of the expected steady state occupancy level for each of the nursing units. The ratio of this steady state to the number of beds in a unit proved to be an excellent predictor of the frequency of internal overflows for that nursing unit, with the frequency becoming large when the ratio exceeded 80%. [ABSTRACT FROM AUTHOR]