The value of home monitoring of peak expiratory flow (PEF) as part of an action plan for asthma management in children and young adults is uncertain. We sought to determine whether home recording of PEF benefited asthma management and whether any contribution was affected by the severity of the asthma. Twenty-eight children and young adults with asthma of different severity (mean age 14 yrs; 95% confidence interval (95% CI) 12-16 yrs) recorded their symptoms, drug consumption and PEF twice daily for a mean of 82 days over a 12 week period, and attended the laboratory every 2 weeks for measurement of lung function. The number of individual patients with significant correlations for laboratory lung function tests compared with ambulatory PEF and diary scores averaged over the preceeding 2 weeks was low in all severity groups. When measured in the laboratory, PEF meter readings correlated poorly with PEF measured by spirometry. The proportion of patients with significant correlations for PEF, symptoms and rescue bronchodilator use on a day-to-day basis was 70-80% in the group of severe asthmatics and significantly less in the mild asthmatics. In a subgroup of 14 patients who were sick on a mean of 19 days, the mean difference in PEF between well and sick days was 14% of predicted. Diurnal PEF variation correlated poorly with other parameters in all groups. It is concluded that PEF monitoring adds little to daily recording of symptoms and bronchodilator use in the management of young patients with severe asthma, and it is too insensitive to register meaningful clinical changes in those with milder asthma.