Liver glycogen storage diseases (GSD) are disorders associated with severe dyslipidaemia which can induce cell membrane alterations. Reduced erythrocyte membrane fluidity has been associated with ischaemic cardiovascular disease. Our study has been designed to investigate membrane erythrocyte fluidity, and to determine its lipid composition and peroxidation parameters. Membrane erythrocyte fluidity has been studied by electron spin resonance (ESR) with two fatty acid nitroxide probes (5NS and 16NS). Twenty-five GSD cases aged 1-27 years and 15 controls aged 1-28 years were included. The erythrocyte membrane of GSD patients appeared less fluid with the two probes (P < 0.001). The membrane fatty acid pattern explained this reduced fluidity. Patients showed a relative saturated fatty acid (SFA) increase and polyunsaturated fatty acid (PUFA) decrease which induced lower PUFA/SFA ratio than in controls. We have provided evidence that the PUFA decrease was independent of the oxidative process. These findings should be taken into account for the management of the dietary treatment of GSD patients.