With the aim of establishing the underlying biochemical lesion in myotonic dystrophy, the lipids of plasma and of the membranes of erythrocytes and cultured skin fibroblasts were investigated in patients with this disorder and normal age- and sex-matched controls. No qualitative or quantitative differences could be detected in the lipid constituents of either plasma or erythrocytes from patients and controls. Similar growth characteristics and levels of sodium [1-14C]acetate incorporation were found in skin fibroblasts from both groups when grown in medium supplemented with either normal serum or delipidated serum. It is concluded that despite the previous evidence that drug induced myotonia results from a primary disturbance of membrane lipids, and suggestions that myotonic dystrophy might have a similar basis, our results did not support this hypothesis. They do not however exclude a primary membrane enzyme abnormality as the basic defect for the disorder.