We assessed whether peak expiratory flow monitoring added to a self-management education programme reduced morbidity and improved pulmonary function and adherence to treatment in 100 asthma patients (aged 17-65 years) with adequate treatment and regular 1-year follow-up. Patients randomized to the experimental group used peak expiratory flow readings as the basis for their therapeutic plan coupled with educational intervention, whereas patients in the control group received the same educational intervention and used symptoms only to guide self-management. Morbidity parameters, functional status and adherence to medical regimens improved in both groups, although the percentage of patients with satisfactory adherence was significantly better in the group with peak expiratory flow monitoring (83%) than in controls (52%) (P = 0.05). The multivariate analysis showed that severity of asthma (odds ratio 9.28, 95% confidence interval 1.87-45.96, P = 0.006 for moderate asthma) and type of self-management education programme (odds ratio: 6.19; 95% confidence interval: 2.04-18.81; P = 0.001 for the use of peak expiratory flow readings) were the only independent predictors of adherence to treatment. However, a statistically significant association between adherence and use of peak expiratory flow monitoring was only found in patients with moderate asthma (P = 0.0009). We conclude that peak expiratory flow monitorization in optimal conditions (adequate medical regimen, individualized self-management education and regular follow-up) showed a beneficial effect on adherence to prescribed regimens only in patients with moderate asthma.