The influence of extrahepatic neoplastic disease on the biotransformation of theophylline was assessed by comparing the pharmacokinetic and metabolic profile of the drug in six patients with advanced breast or bronchial carcinoma, without detectable liver metastases, and in six appropriately matched control subjects. Each subject was given a single dose of theophylline (5 mg/kg) in oral solution; blood and urinary samples were collected for up to 24 h after dosing. Theophylline was absorbed rapidly in all subjects and within 2 h reached comparable peak concentrations in both groups (cancer patients: 57.8 +/- 14.4 mumol/l; controls; 65.0 +/- 10.6 mumol/l; N.S., means +/- s.d.). No significant differences were observed between cancer patients and controls for theophylline apparent volume of distribution (0.44 +/- 0.07 vs 0.40 +/- 0.06 l/kg), total body clearance (40.8 +/- 12.8 vs 34.8 +/- 13.0 ml kg-1 h-1) and elimination half-life (8.0 +/- 1.6 vs 8.5 +/- 1.8 h). The excretion of the major metabolites 3-methyl-xanthine and 1,3-dimethyl-uric acid was also very similar in the two groups. These data do not provide any evidence for an altered rate or pattern of theophylline biotransformation in patients with advanced extrahepatic neoplastic disease.