Enprofylline, a xanthine-derivative shown experimentally to lack universal adenosine receptor antagonism, has been examined in patients with partly reversible, chronic, obstructive lung disease. Significant bronchodilation was produced by enprofylline 2 mg/kg, giving a peak plasma concentration of 3.0 +/- 0.6 microgram/ml (mean +/- SD). A dose of 2 + 4 mg/kg dilated the bronchi at least to the same extent as theophylline 9.2 +/- 0.9 mg/kg (plasma level 18.5 +/- 4.7 micrograms/ml). Neither at the low nor at the high dosage (2 +/- 4 mg/kg), giving plasma concentrations of 8.5 +/- 1.4 microgram/ml, did enprofylline produce theophylline-like CNS effects, such as restlessness and tremor, but it did exhibit some of the innocuous side effects expected with xanthine derivatives, such as epigastric discomfort and headache. The comparison with theophylline was limited because different dosage forms had to be used (solution an tablets), which for example, resulted in different absorption rates. Nevertheless, the present findings indicate enprofylline to be potent bronchodilator in patients with obstructive lung disease, suggesting that adenosine-receptor antagonism is not involved in the bronchodilator effects of xanthines.