The R141H mutation in the PMM2 gene is the most frequent mutation in type Ia of the congenital disorders of glycosylation (formerly carbohydrate-deficient glycoprotein syndromes)(CDG-Ia). However, it has never been observed in the homozygous state. Homozygosity for this mutation is probably incompatible with life. In this study, we determined the frequency of R141H in two normal populations: in neonates of Dutch origin 1/79 were carriers, whilst in the Danish population, a carrier frequency of 1/60 was found. These figures are clearly in disequilibrium with the frequency of CDG-Ia that has been estimated at 1/80 000 to 1/40 000 in these populations. Haplotype analysis of 43 patients with the R141H mutation of different geographic origins indicated that the R141H is an old mutation in the Caucasian population. Based on the new data, the disease frequency has been calculated at 1/20 000 in these populations. It is concluded that the disease is probably underdiagnosed.