The population distribution of bronchial responsiveness to inhaled histamine was examined in 300 randomly selected caucasian college students (aged 20 to 29 years). Bronchial responsiveness was measured as the histamine threshold, defined as the concentration producing an FEV1 fall greater than 2 SD below the mean of five prehistamine FEV1 measurements. The cumulative prevalence of asthma was 9.3 percent, including 2.7 percent with current asthma, 3.3 percent with asthma following allergen exposure only, and 3.3 percent with remote asthma. Allergic rhinitis was present in 10.7 percent; nonallergic rhinitis in 16.3 percent; 63.7 percent had neither asthma nor rhinitis. Histamine threshold ranged from unmeasurable (greater than 8 mg/ml) in 36 percent to 0.125 mg/ml in 0.3 percent. The distribution of histamine threshold values in the responsive range was unimodal, the asthmatic subjects representing a subgroup within the hyperresponsive distribution tail rather than a separate distribution peak. Examination of the FEV1 response to 8 mg/ml showed a range between 2.8 SD increase and 100 SD reduction; the population distribution of this variable was unimodal and log normal. We concluded that there is a continuous unimodal log normal distribution of bronchial responsiveness to inhaled histamine in a random human population. Rather than representing a separate (bimodal) peak or a sharp cutoff (of a unimodal tail), the asthmatic subjects show substantial overlap with the remainder of the population.