A 6 year old Portuguese boy with dilated cardiomyopathy had abundant ragged red fibres in muscle (20% of total) and severe lactic acidosis. Molecular genetic analysis showed the A to G transition in the mitochondrial transfer RNALeu(UUR) gene at nt 3243 ("MELAS mutation"), which accounted for 88% and 68% of the total mtDNA in his muscle and blood, respectively. Molecular studies in blood from 16 maternal relatives identified lower percentages of the mutation only in the oligo-symptomatic mother and brother. This case reinforces the notion that cardiomyopathy can be the presenting and predominant clinical expression of the A3243G mutation.