The prevalence rate of severely mentally and physically disabled children (SDC) aged 6-15 years in Okinawa prefecture on 1 May 1989 was 0.74/1000 (143/192,038) according to Oshima's classification, compared with 0.89/1000 (170/192,038) according to the Ministry of Education's classification with minor modifications. The number of children in region classes 1, 2, 3 and 4 of Oshima's classification for SDC were 100, 34, 6 and 3, respectively. The difference (n = 27) between the total numbers of SDC according to the two classifications was mainly because of 21 children categorized as "walking with support' who were included as SDC according to the Ministry of Education's classification but not as SDC according to Oshima's classification. Only region class 1 of Oshima's classification corresponded with region class 25 of the Ministry of Education's classification. The results of the present study indicate that the differences between the two definitions of SDC affect the reported prevalence rates of SDC. Therefore, changing patterns in the prevalence of SDC should be assessed by serial surveys using the same method in each district.