To investigate the attitudes and practices of GPs towards the use and monitoring of theophyllines, a questionnaire was mailed to all 479 GP principals in Nottinghamshire with one reminder to non-responders. Valid responses were obtained from 49% (236). Theophyllines were regarded as first-line or frequent second-line bronchodilators by 40% (94 of 234) of GPs in chronic bronchitis, 24% (56 of 234) in nocturnal asthma, and by 6% (15 of 234) in chronic stable asthma, although only 1% (2 of 234) of GPs regarded theophylline as strictly first-line therapy in chronic stable asthma. Theophylline levels were never checked by 76% (172 of 225) of practitioners at the start of therapy, or by 48% (110 of 231) during long-term therapy; half (110 of 217) could identify only up to 50% of a series of common factors affecting theophylline levels. There was no relationship between a score of theophylline knowledge and prescribing habits in chronic bronchitis, but doctors who checked levels scored significantly higher. Although theophyllines are being used in general practice for broadly sensible indications, this survey highlights aspects of prescribing that may be compromising both safety and effectiveness. Patients are likely to benefit if the need to check levels and the factors affecting levels become better known.