Since management plans based on peak flow measurements are increasingly used, the relationship between peak flow rate and ensuing symptom frequency is of particular interest to the treatment of asthma. The objective of this study was to examine to what extent morning peak flow was related to symptom frequency during the day.In 168 out of 307 randomly selected adult asthmatics from a general practice population, adequate recordings of morning peak flow (amPF) and symptom frequency during the day (DSF) were obtained for 28 days. In each individual, the relationship between these two variables was studied and the mean values of these two variables over 28 days were calculated. The relationship between the means of the variables was also examined for the group as a whole.In individuals, the correlation between amPF and DSF varied widely; only 16% of patients had a good relationship (Pearson r -1 to -0.5). For the whole group, the relationship between mean amPF and mean DSF best fitted a curvilinear model (r -0.6). This was unaffected by age, sex or the use of inhaled steroids.For the majority of asthmatics, morning peak flow may be an unreliable predictor of expected symptoms during that day. Reliance on peak flow measurement as a tool enabling asthmatics to manage their asthma on a daily basis needs to be reconsidered.