The discovery of a point-mutation, adenine-to-guanine, at position 985 in the gene coding for MCAD (G985), gave the basis for an easy and specific polymerase chain reaction test. We tested the specificity of such a PCR based assay and detected correctly G985 and A985 in sequence verified cDNA clones. We showed that the G985 mutation is present in genomic DNA from 48 of 50 patients with confirmed MCAD deficiency, originating from various European countries, Australia and the USA. On the basis of this high frequency of the G985 mutation among patients, we improved and optimized the assay with respect to reliability and convenience for routine diagnostic and screening purposes. As little as 2 microliters blood from filter-paper blood-spots (Guthrie spots) is sufficient for the test.