We compared the use of telemonitoring in patients with chronic obstructive pulmonary disease (COPD) and adult patients with cystic fibrosis (CF). Seventy patients (51 CF and 19 COPD) were enrolled in two studies of six months' duration. Patients used a personal data assistant (PDA) attached to a spirometer to score symptoms and to perform daily spirometry. Criteria for diagnosis of exacerbations of COPD and CF were pre-defined. When exacerbations were detected, patients were offered treatment according to a pre-designed protocol. Thirty-two (63%) CF patients and one (5%) COPD patient withdrew from the studies due to lack of adherence to daily recording. For those who remained in the study, COPD patients recorded more study days (139) than CF patients (113), P = 0.03. The median number of exacerbations detected during the study was greater in COPD than in CF patients, although this was not significant. The median number of device-detected exacerbations in the COPD group was significantly greater than in the CF group, P = 0.024. When compared to a parallel period in the previous year, the number of hospitalisations for COPD exacerbations was reduced, whereas the number of intravenous antibiotics in CF patients did not differ. Adherence to telemonitoring was much greater for COPD than CF patients and the results appear to be more favourable for COPD patients than for CF patients.