Variability in demand for hospital services may have important effects on hospital costs, but this has been difficult to examine because data on within-year variations in hospital use have not been available for large samples of hospitals. We measure daily occupancy in California hospitals and examine variation in hospital utilization at the daily level. We find substantial day-to-day variation in hospital utilization, and noticeable differences between hospitals in the amount of day-to-day variation in utilization. We examine the impact of variation on hospital costs, showing that increases in variance are associated with increases in hospital expenditures, but that the effects are qualitatively modest