A study was performed to verify that the prevalence of alcohol abuse and dependence in Formosan aborigines differs from that of Taiwanese (Chinese Han people), using analysis of aldehyde dehydrogenase (ALDH) isozymes and flush patterns on randomly sampled 70 Atayal, 66 Paiwan, 61 Yami and 94 Taiwanese subjects were studied. The activity of an isomer of ALDH having a low Km (ALDH-I) in hair roots was analysed by isoelectric focusing assay. The subjective experience of flushing response after alcohol ingestion was assessed. Results showed that the rate of ALDH-I deficiency in Taiwanese (51.1%) was significantly higher than in aborigines, i.e., 6.4%, 3.9%, and 0% in Atayal, Paiwan, and Yami subjects, respectively. The percentage occurrence of ALDH-I deficiency and prevalence of alcohol dependence in Taiwanese and aborigines were negatively correlated. The predominant pattern of self-reported flush response after alcohol use among aborigines was of slow onset. The flush response to alcohol ingestion was examined in relation to aldehyde metabolizing enzyme. Since alcohol sensitivity is an important factor in the development and maintenance of the alcohol ingestion habit in humans, our results support the hypothesis that there is a biological basis in the different rates of alcohol abuse and dependence among different ethnic groups.