The effect of parental smoking on childhood asthma was investigated in which data from 302 asthmatic and 433 healthy children aged 1 to 12 years, were studied. All asthmatic patients received prick tests for common allergens. A significantly higher number of heavy parental smokers was found in asthmatic children under 6 years of age with negative prick tests (P = 0.02). A stepwise logistic regression was performed in order to verify interactions between parental smoking and other variables. It is concluded that parental smoking is an important risk factor for "prick test negative" asthmatic children aged 6 years or less.