The bronchial hyperreactivity was examined in a follow-up of 206 children having had obstructive bronchitis before two years of age. The acetylcholine challenge was done with 0.5% and 1% Pharmalgen solutions. The bronchial hyperreactivity was established over a 20% decrease of PEF and/or FEV1 values. Summarizing the results produced by 0.5% and 1% solutions a significant difference could be found in the bronchial hyperreactivity between the follow-up and control groups. Similarly the PC20 (provocative concentrations) values were suitable for differentiating the two groups. Upon physical exercise, EIB occurred in 2%. With the skin prick test (SPT) by 10 allergens 3+ or 4+ positivity was found in 17% of the children. Between the bronchial hyperreactivity and SPT positivity a significant correlation could be established.