Twenty operated cases of angiographically unrecognized microaneurysm (AUM) have been analysed with special reference to intra-operative observations and clipping-technique. Among the patients with intracranial aneurysms that the authors' facility has operated upon, the incidence of asymptomatic incidental AUM that was 2 mm or smaller amounted to 3.7%. Thirteen cases of AUM were found on the middle cerebral artery; four AUMs arose from the M1 portion, four from the bifurcation, and five from the second bifurcation. Sixty percent of AUMs were recognized on the parent arteries of ruptured aneurysms. In 90% of cases the AUMs were broad based in shape and in 70% of cases exhibited a thin-walled neck and a thin-walled fundus. Intra-operative findings revealed four reasons why AUMs were not visible in the pre-operative angiograms: (1) the AUM was sandwiched between two arteries; (2) the AUM was completely hidden by a contiguous large or giant aneurysm; (3) the AUM was diagnosed by pre-operative angiogram as a bleb of the contiguous aneurysm; (4) the AUM was not visible on angiograms because the height of the AUM was extremely low. Twelve cases of AUM were successfully clipped using four different clipping techniques; (1) clipping parallel to the bifurcation in four, (2) clipping parallel to the parent artery in four, (3) pinch-clipping in two of the cases, and (4) cross clipping in two of the cases. The other eight cases were wrapped and coated. AUMs may be present during the direct operation of intracranial aneurysms and in intravascular surgery. Neurosurgeons and neuroradiologists need to explain the possible existence of AUMs to patients and their families.