Dietary intervention is the first treatment step in management of hyperlipidaemia, but there are few objective criteria of compliance. Whether intensive dietary intervention would produce a detectable change in erythrocyte membrane fatty acid composition which could be used as a marker of compliance was examined in 31 new hyperlipidaemic patients. Over a 6 month period, body mass index fell from 29.0 to 26.9 kg/m2 (P < 0.001) and total cholesterol by 19% from 8.16 to 6.58 mmol/l (P < 0.001). The energy derived from fat was reduced from 38.5% to 29.6% (P < 0.001), and the ratio of dietary polyunsaturated to saturated (P:S) fatty acids in the diet increased from 0.45 to 0.66 (P < 0.01). Small but significant changes were recorded in several red cell membrane fatty acids, and the P:S ratio increased from 0.91 to 1.13 (P < 0.001). It would appear, therefore, that red cell membrane changes parallel dietary changes and hence are a potential marker for compliance with dietary changes.