Beginning in November 1988, Phase II compared initiation of thrombolysis with tissue plasminogen activator (TPA) in the field to initiation in the hospital. During Phase II, which lasted for 24 months, one half of the patients meeting the case definition received paramedic administration of intravenous thrombolytic therapy and one half received the same pre-hospital diagnostic steps but treatment administered after transport to the hospital. The 360 patients with symptoms for six hours or less, no risk factors for serious bleeding, and ST-segment elevation, were selected by paramedics and a remote physician for inclusion in the trial. They were allocated to aspirin and alteplase treatment initiated before or after hospital arrival. Intravenous sodium heparin was administered to both groups in the hospital. Nineteen hospitals in the Seattle and King County areas participated. The primary endpoint was a ranked composite score (combining death, stroke, serious bleeding, and infarct size). The relation between time to treatment and outcome (composite score, infarct size, ejection fraction, and mortality) was also assessed.