The polymorphic methylenetetrahydrofolate reductase (MTHFR) gene polymorphisms C677T and A1298C cause mild hyperhomocysteinemia, not only in homozygotes for C677T, but also in compound heterozygotes for C677T/A1298C. The aim of this study was to determine allelic frequencies of the polymorphic MTHFR gene C677TA1298C. In this regard, we have investigated the allelic frequencies of C677T and A1298C polymorphisms of the MTHFR gene in 1684 randomized individuals around Turkey. DNA samples isolated from peripheral blood samples of randomized individuals were analysed. The study population consisted of 1004 females and 680 males. The frequency in Turkey of the C677T was 42.9%; of C677C, 47.4%; and of T677T, 9.6%. The frequency in Turkey of A1298C was 43.7%; of A1298A, 46.3%; and of C1298C, 10.0%. The allelic frequencies of the T allele of MTHFR 677 and the C allele of MTHFR 1298 were 33.34 and 33.16%, respectively. The frequency of C677T/A1298C compound heterozygosity is highest in Turkey (21.6%), as compared to Canada (15%), the United States (17%) and The Netherlands (20%). Copyright (C) 2004 John Wiley Sons, Ltd.