BACKGROUND: An adequate erythrocyte function is vital for tissue oxygenation and wound healing. The erythrocyte membrane phospholipid composition plays an important role in erythrocyte function and administration of omega-3 fatty acids may provide a means to improve it. OBJECTIVE: To investigate peri-operative erythrocyte function and effects of oral omega-3 fatty acids in morbidly obese women undergoing gastric bypass surgeryMETHODS:Fifty-six morbidly obese women undergoing laparoscopic Roux-en-Y gastric bypass (LRYGB) surgery were randomized between a low calorie diet (LCD) during 2 weeks or oral omega-3 poly-unsaturated fatty acids (n-3 PUFAs) and a normal diet during 4 weeks. Peri-operative blood samples were analyzed with the Lorrca MaxSIS Ektacytometer for erythrocyte deformability and aggregability. RESULTS: There were no significant differences in erythrocyte function between the groups at any time point. Only erythrocyte aggregability parameters were affected by surgery. At six month follow-up, aggregation index (AI) and cholesterol, glucose and insulin were significantly improved. CONCLUSIONS: In this study, oral Omega-3 supplementation did not affect erythrocyte function compared to a LCD. Six months after surgery a significant improvement in AI and metabolic parameters was observed in both groups, contributing to a reduction in the risk at thromboembolic and cardiovascular complications.