A quantitative methodology that computes the capacity of hospitals to operate on injured people in the aftermath of a seismic event is presented. The model incorporates variables such as organizational and human preparedness, damage to buildings, the number of operating theatres, the amount of supplies, and the duration of a surgical treatment. The damage to buildings is assessed probabilistically using fragility curves while the other variables are calculated deterministically. The model is applied to a selection of 41 existing hospitals in the Lima metropolitan area. The results highlight that the investigated health facilities possess a low hospital treatment capacity in the aftermath of a seismic event. This is due to a high expected seismic damage level of the analysed hospital pavilions and insufficient autonomy in essential resources such as water, electrical power, medical gases, and medicines. Given the low hospital seismic performance, external aid is necessary in the case of immediate needs, for example support from the private sector and field hospitals. Among the preventive measures to improve the hospital treatment capacity, structural and non-structural retrofitting of the existing structures is useful, but increasing the backup of essential resources and the construction of new operating rooms can also be efficient. We also find that hospitals are concentrated in the central area of the metropolis where other health facilities with adequate backup of essential resources are located, highlighting the gap in hospital treatment capacity between the centre and suburbs.