Theophylline clearance is altered by many drugs and diseases that may be associated with undernutrition. Therefore, we studied theophylline clearance in 12 undernourished [body mass index (BMI) less than 19] and 12 well nourished asthma patients (body mass index more than 19). Uncoated theophylline tablet (200 mg, 300 mg dose 5 mg/kg approx) was administered orally to each asthma patient after 12 hours overnight fasting. Serum theophylline concentrations were estimated after 2, 4, 6, 8 hours of drug administration and it was calculated from log conc: time curve. Undernourished asthma patients had a mean theophylline clearance of 85.6 (SE = 6.2) ml/hr/kg while it was 125.6 (SE = 3.8) ml/hr/kg in well-nourished asthma patients. The difference between two groups was highly significant (p < 0.01). We conclude that theophylline clearance is significantly lower in undernourished asthma patients and they will require a lower maintenance dose of theophylline.