In individuals with sickle cell disease, a variable number of irreversibly sickled cells (ISC) is present that may contribute to the pathophysiology of sickle cell anemia. The present study was undertaken to determine the possible role of membrane lipid peroxidation in the genesis of ISC. After 24 hr of simple aerobic incubation, sickle cells accumulated 2–3 times more malonyldialdehyde (MDA), an end product of lipid peroxidation, than normal cells. To assess the possibility of peroxidative damage in ISC in vivo, ISC were separated from sickle blood using Stractan density gradients. Lipid extracts of the untreated ISC-enriched fraction of sickle blood showed significant fluorescence and contained a novel phospholipid:MDA adduct that was not seen in control cells. Taken together, these observations suggest that ISC have previously undergone lipid peroxidative damage and the accumulation of MDA in vivo.