With theory and method of genetic epidemiology, data of 641 nuclear families on asthma were analysed, and the results showed that the history of parents asthma was one of risk factors about offsprings asthma, and the risk was 4.0-8.0 times higher (P < 0.01), compared with the one of offsprings of parents with no asthma. The effect of parents with asthma on the risk of sons was the same as one of daugters. The result declared that the airway responsiveness of parents did not relate to the one of offsprings. The results suggest that the genetic factors play an important role in the asthma.