A suggested model for the development of an urban based prehospital emergency care system is described. Factors considered in the planning and development include: 1) demand for services, projected and actual; 2) analysis of costs; 3) design and maintenance of the delivery system; and 4) establishment of the evaluation mechanisms. Over one year's experience and 1,144 mobile intensive care unit (MICU) calls in a densely populated urban setting with over 500,000 persons are reported. During the peak 8-hour period, predetermined dispatch categories were employed to activate one MICU operating in conjunction with three conventional ambulances. This partial conversion imparted MICU capability to the entire system at an 11 per cent increase in the ambulance budget. MICU calls averaged 4.5 per 8-hour peak shift and took 45 minutes each.