In a single-blind cross-over study the effect of oral treatment with pantethine on plasma and platelet lipid composition was evaluated in 20 patients with dyslipidaemia (7 IIa, 7 IIb and 6 Iv type). In plasma significant decreases of total cholesterol and triglycerides with increase of high density lipoprotein-cholesterol were observed. In platelets pantethine treatment significantly reduced phospholipid and cholesterol content. In addition gas-chromatographic analysis showed a reduction of saturated and monounsaturated and a relative increase of polyunsaturated fatty acid content of platelet phospholipids. A selective relative increase was observed of some n-3 polyunsaturated fatty acids like eicosapentaenoic and docosahexaenoic acid whereas arachidonic acid decreased. The present study indicates a favourable influence of pantethine not only on plasma but also on platelet lipids which could be of value in delaying the development of atherosclerosis in dyslipidaemic patients.