BACKGROUND: Early detection and treatment of patients with asthma or chronic bronchitis who have a rapid annual decline in lung function is essential in order to improve their long-term prognosis. This annual rate of decline can be assessed accurately by monitoring the forced expiratory volume in one second (FEV1) which is a routine procedure in hospital respiratory laboratories but not in general practice. General practitioners usually measure patients' peak expiratory flow rate (peak flow) to evaluate lung function. If annual decline in lung function can be assessed by monitoring peak flow, this method could be used in general practice for detecting patients at an early stage who have a rapid decline. AIM: A study aimed to investigate the long-term correlation between FEV1 and peak flow among a group of patients in Nijmegen, the Netherlands. METHOD: FEV1 and peak flow were monitored in 53 patients with moderate asthma and 78 patients with moderate chronic bronchitis over four years. FEV1 was measured in a laboratory once every six months and peak flow was measured by patients once a week. The correlation between the two sets of measurements was studied for each patient. RESULTS: Four-year data for 83 of the 131 patients were analysed; the other 48 patients received inhaled steroids during the second half of the study period so their data were not considered for all the analyses. Of the 83 patients, 35 (42%) showed a decrease in both FEV1 and peak flow. Thirty six patients (43%) showed a decrease in FEV1 and an increase in peak flow. Four patients (5%) showed an increase in FEV1 and a decrease in peak flow and eight patients (10%) showed an increase in both rates. Approximately similar results were seen in a separate analysis of all 131 patients during the first two years of the study. CONCLUSION: No long-term correlation was found between FEV1 and peak flow. Peak flow is not capable of detecting annual decline in lung function. Therefore it cannot be used to detect patients with asthma or chronic bronchitis who have a rapid annual decline in lung function. Spirometers, which measure peak flow and FEV1, could be used in general practice. These would allow general practitioners to continue measuring peak flow in order to assess short-term changes in lung function while providing an important means for monitoring FEV1 to assess long-term changes in lung function.