The authors studied the epidemiology of spina bifida in Tottori Prefecture, Japan, from 1976 to 1995. Thirty-four patients (16 men and 18 women) were registered in this study. Consanguineous marriages, familial occurrence, and abnormalities in prenatal history were not observed. The incidence rate in the entire prefecture and in the eastern, central, and western regions was 0.234, 0.148, 0.425, and 0.230 per 1,000 live births, respectively. The incidence rate in the central region was greater than that in the eastern region with statistical significance (P < 0.05), but the cause of the cluster is unknown. The incidence rate of 0.234 per 1,000 live births for 20 years is compatible with the previous two studies of 1922-1940 and 1948-1954 in Japan. Such apparently stable trends suggest that environmental factors have affected the Japanese less than genetic factors. Seasonal variations are not demonstrated.