Characteristics and prognostic relevance of morning dip of peak expiratory flow rate (PEFR) were evaluated in stable asthmatic subjects. Among 246 outpatients monitored four times daily for two weeks, 38 (group A) showed a significant difference between morning reading of PEFR and each of the others; they were compared to 38 randomly selected patients (group B) not showing morning dip in PEFR. Less frequent seasonal course, extrinsic pathogenesis, and sensitization to mites characterized group A; starting airflow limitation was more severe in those with morning dip, but no significant difference between mean PEFR measured throughout two weeks was found. At 6 to 12 weeks, morning dip was not found in 19 of 38 subjects in group A and appeared in seven of 38 subjects in group B, with no clearcut relationship to treatment being evident. At 25 to 104 weeks, no significant difference between therapeutic requirements and the forced expiratory volume in one second was detected; therefore, unlike the short-term, morning dip is not a risk factor for worse long-term prognosis.