The frequency of the methylenetetrahydrofolate reductase enzyme (MTHFR) C677T mutation was determined using polymerase chain reaction (PCR) and with measurement of plasma total homocysteine (tHcy), folate, vitamins B6, B12 and disease severity in 102 SS children from Yemen. The homozygous TT genotype for MTHFR C677T was present in 2% (2/102), and heterozygous CT in 10.8% (11/102), giving an allele frequency of 7.35%. The T allele was not associated with raised plasma tHcy or increased disease severity. The mean [+/-SD (standard deviation)] tHcy was 2.8 +/- 1.7 micromol/L, increased with age and was highest in children >10 years (3.6 +/- 2.5 vs. 2.5 +/- 1.2 micromol/L, p <0.05). Whole blood folate and plasma vitamin B12 levels were normal or elevated, and 4% had vitamin B6 deficiency. In Yemeni children with sickle cell disease the frequency of the MTHFR C677T mutation was not higher than expected in the general population and was not associated with disease severity.