MCAD is the commonest inherited disorder of fatty acid oxidation. We have sought for and studied 21 affected children from 18 families within the UK. In 14 families the children are homozygous for the G985 mutation. In three families the children are compound heterozygotes for G985 and thus carry another and unknown mutation. In one family the child does not carry the G985 mutation on either allele. The carrier incidence of the G985 mutation is 1 in 68, which suggests that the natural history of MCAD deficiency deserves further study.