Eighteen asthmatics, aged 7 to 43, rated their breathing on a scale of 0 to 5 (worst to best). They were then informed of their predicted normal peak expiratory flow rate (PEFR) and instructed in proper PEFR measurement. Each subject then estimated and measured PEFR under physician supervision and subsequently rated breathing, estimated and measured PEFR at home twice daily recording the observations. At the initial home observation, the correlation coefficient (r1) between estimated and measured PEFR was .98. After ten observations r1 was greater than or equal to .97 in all 18 subjects (group mean .99). The r1 remained unchanged in those subjects who completed 28 and 42 observations. The r1 was .98 when only the observations where measured PEFR was less than 20% predicted were considered, demonstrating the validity of the relationship during abnormal expiratory flow. Multiple regression analysis showed no significant trend in the difference between estimated and measured PEFR over time after the initial observation. Of 12 subjects, 4 were significant underestimators of PEFR and 3 were overestimators, but the magnitude of either tendency was relatively small. The correlation coefficient (r2) between a subjective verbal breathing score and measured PEFR ranged from -.23 to .92 on an individual basis over the three time periods, with statistically significant group mean values of .37, .39, and .45 at 10, 28, and 42 observations, respectively. The r2 for observations where measured PEFR was abnormal was only .14 and not significant.(ABSTRACT TRUNCATED AT 250 WORDS)