SummarySustained-release theophylline ('Nuelin SA) given twice daily in a weight-related dose to 16 asthmatic patients maintained theophylline serum levels within the therapeutic range. On Day 14, these levels were associated with a statistically significant increase in PEFR (p<0.01) from the mean pre-treatment values of 271 litres/min. The mean trough PEFR value was 20% greater and the mean PEFR at noon 41% greater than the pre-treatment value. There was a reduction in nocturnal and early morning asthma symptoms. Measurement of drug concentration in saliva is attractive since it is non-invasive. In this study, the correlation of theophylline levels in saliva and serum was good (r = 0.95). It would seem that once steady-state theophylline serum and saliva levels have been measured, monitoring of theophylline treatment using saliva measurements may be possible within the limitations reported.