PURPOSE: To investigate the astigmatic changes induced by biaxial microincisional cataract surgery with enlargement of one incision to accommodate a foldable standard lens. METHODS: In a prospective controlled trial one eye each of 50 patients was evaluated. Two temporal microincisions were applied and the right-hand incision was enlarged to 2.8 mm to allow for the insertion of the lens. Based on preoperative and postoperative keratometry readings, vector analysis was performed to calculate the surgically induced astigmatism (SIA) and the flattening effect of each of the two incisions at their meridia. To assess the degree of measurement error, a non-surgical control group underwent keratometry over the same time frame and vector analysis was used to calculate the same indices based on the variation in the keratometry readings. RESULTS: Six weeks postoperatively, mean SIA was 0.66 +/- 0.08 D in the study group and 0.51 +/- 0.05 D in the non-surgical control group (P = 0.041). At the right-hand incision, a flattening of 0.34 +/- 0.09 D was measured in the study group as opposed to a steepening of 0.03 +/- 0.06 D in the control group (P < 0.001). At the smaller left-hand incision, steepening effects of 0.28 +/- 0.08 and 0.05 +/- 0.06 D occurred, respectively (P = 0.011). CONCLUSION: Biaxial microincisional cataract surgery with enlargement of one incision to 2.8 mm is not astigmatically neutral, demonstrating a statistically significantly larger SIA than that attributable to measurement error.