A double-blind crossover study has been performed in 14 patients with moderately severe chronic asthma to compare the bronchodilator efficacy of two dosage regimens of intravenous enprofylline (high dose = 2 mg/kg bolus and 1 mg/kg/hour infusion; low dose = 1 mg/kg bolus and 500 micrograms/kg/hour infusion) with aminophylline (5 mg/kg/bolus and 500 micrograms/kg/hour infusion) and placebo. The bolus injections were given over 20 minutes and infusion over 160 minutes. Twelve subjects completed the study. High dose enprofylline was more effective than aminophylline in increasing PEF (P = 0.008) and FEV1 (P = 0.004). Low dose enprofylline and aminophylline were of similar efficacy. Side-effects, notably headaches and nausea, were more common with enprofylline; three out of 14 subjects receiving the high dose regimen developed severe nausea. The plasma enprofylline levels achieved with the high dose regimen were greater than anticipated. Further studies are required in acute severe asthma to clarify the therapeutic role of intravenous enprofylline and the most appropriate dosage regimen.