The histamine inhalation provocation test (HIP) was followed during 5 weeks. This test was performed four (1 patient) or five times under standardized conditions in a group of 21 asthmatic patients. The HIP was performed with increasing concentrations of histamine phosphate produced by a 'Wiesbadener Doppelinhalator'. The HIP was called positive if there was a drop of 15% of the initial FEV1. 12 out of 21 patients (57%) showed a sufficient reproducibility. For the whole group of patients there hardly was a variability of the mean absolute concentration of histamine to produce a positive test, but individual patients showed a wide spread. Furthermore, the correlation between the initial value of the FEV1 and the bronchial response seemed to be highly significant (p less than 0.0001). It is concluded that the HIP is not always reproducible within a short period and that the hyperreactivity of the airways is correlated to the initial value of the FEV1.