Topical timolol given for the treatment or chronic simple glaucoma may cause unrecognised bronchospasm among elderly people. We recruited 80 patients aged over 60 years, who were without a history of airways disease and already used timolol, into a randomised crossover study comparing the effects on spirometry and exercise tolerance of changing to betaxolol or dipivefrine therapy. Results showed an increase of 13% and 8% in mean peak flow rate and forced expiratory volume in 1 s (FEV1), respectively, when using betaxolol; and of 14% and 11% when using dipivefrine. There was also improved exercise tolerance with both agents. More than a quarter of the patients showed at least a 15% improvement in FEV1 when changed from timolol. Analysis of enrolment symptoms and response to nebulised salbutamol failed to produce a method of identifying these patients. Timolol may impair respiratory function and exercise tolerance of elderly patients even if they have no history of reversible airways disease.