OBJECTIVE: To investigate the efficacy of capacity alert calls in reducing acute hospital overcrowding through addressing rising occupancy, high patient throughput and increased access block. METHODS: Retrospective analysis of 24 months of in-patient, emergency department, and capacity alert call log data from a large metropolitan public hospital in Australia. The analysis explored statistical differences in patient flow parameters between capacity alert call days and other days including a control case set of days with statistically similar levels of occupancy. RESULTS: The study identified a significant (P<0.05) reduction in occupancy, patient throughput and access block on capacity alert call days. Capacity alert call days reversed rising occupancy trends, with 6 out of 7 flow parameters reporting significant improvement (P<0.05) over the 48 h following the call. Only 3 of these 7 flow parameters were significantly improved 48 h after control case days, confirming value in the alert mechanism and that the results are not a regression toward the mean phenomenon. CONCLUSIONS Escalation processes that alert and engage the whole hospital in tackling overcrowding can successfully deliver sustained improvements in occupancy, patient throughput and access block. The findings support and validate the use of capacity alert escalation calls to manage overcrowding, but suggest the need to improve the consistency of trigger mechanisms and the efficiency of the processes initiated by the capacity alert call.