OBJECTIVE We conducted a study of the prehospital use of inhaled metaproterenol. DESIGN, SETTING, TYPE OF PARTICIPANTS, AND INTERVENTIONS: Advanced life support (ALS) providers were trained with a standardized curriculum to identify patients likely to benefit from prehospital inhaled metaproterenol administration. Unit doses of metaproterenol were used in a small-volume nebulizer. We prospectively included 122 patients in an initial study (71 men; age, 63 +/- 19 years) to evaluate the safety and effectiveness of metaproterenol in the field, and 150 patients (including the original 122) in an additional study to evaluate patient selection criteria. RESULTS The treatments resulted in an increase in peak flows, a decrease in respiratory rates, and no change in heart rates. In 62% of patients, the increase in peak flow exceeded 15%. Wheezing improved in 59% of the patients, worsened in 4%, and did not change in the remainder. Air entry by auscultation improved subjectively in 59% of patients. Mild tremor occurred in 8% of patients, moderate tremor occurred in 1%, and no tremor occurred in the remainder. Significant dysrhythmias did not occur. CONCLUSIONS ALS providers correctly identified patients for this therapy. No technical problems were encountered in the field with this treatment approach. We conclude that ALS providers can be taught to identify patients likely to benefit from inhaled metaproterenol, that inhaled metaproterenol can be administered in the field, and that metaproterenol is both safe and effective when used in the prehospital setting.