In 12 asthmatic children, aged 8-14 years, we investigated the possibility of using a provocation concentration of histamine (PC) causing a 10% or 15% fall in baseline forced expiratory volume in 1 sec (FEV1), instead of a PC20 histamine, in the assessment of bronchial hyperreactivity. Inhalation challenge tests were performed on days 6 and 7 after withdrawal of medication. PC10, PC15, and PC20 were calculated from the dose-response curves. Reproducibilities for PC10, PC15, and PC20 values for both days, determined by Student's t-test, were not significantly different. Correlation coefficients between PC20 and PC15 values on days 6 and 7 and between PC10 values on days 6 and 7 were 0.82 and 0.76, respectively; between PC20 values on day 6 and PC15 values on day 7; and between PC20 values on day 6 and PC10 values on day 7 they were 0.84 and 0.82, respectively. (P less than 0.01 for all r values). The predictive value of PC20 for PC10 and PC15 was determined by a least-squares regression line with 95% confidence intervals. Variances between PC10, PC15, and PC20 were not significantly different on either day 6 or 7. Our data show that a PC10, as well as a PC15, can be used in the assessment of the degree of bronchial hyperreactivity in children with asthma.