Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is recognized as the most common hereditary defect of fatty acid oxidation in humans. Death is the outcome of the first attack in about 25% of cases. A point mutation (A to G [corrected] at position 985) of the MCAD gene represents more than 90% of alleles causing MCAD deficiency. The frequency of this allelic variant exhibits considerable geographical variations. In Spain, where the few diagnosed patients are mostly of Gypsy origin, the frequency is low as occurs in other Southern European countries (1 heterozygote among 200 individuals). Here we have analyzed the frequency of the G985 allele among Spanish gypsies. Heterozygotes were detected at a frequency of 1/17, with a 95% confidence interval ranging from 1/11 to 1/39. This represents the highest G985 rate described so far and calls for preventive measures, such as selective screening in this population.