Diurnal variation of peak expiratory flow was assessed over 10 days in 40 asthmatic out-patients judged clinically to be stable. Thirty patients showed a morning dip in their peak expiratory flow and the frequency with which this occurred appeared to be linked with the amplitude of the dip. The percentages showing large, moderate and insignificant dips were similar to those described in asthmatics hospitalized with an exacerbation. Ten patients were admitted with an exacerbation of their asthma in the preceding 2 yr. Of the 3 non-dippers, 2 assumed a dipping pattern during this admission. Without a continuous peak expiratory flow record, it was not apparent that many of these apparently stable patients were showing high amplitude dips, such as those associated with sudden unexpected death. The relevance of a continuous diurnal peak expiratory flow record in relation to asthma mortality is discussed.