Twenty-one macroaneurysms and related vessels of 19 patients were evaluated morphometrically. Macroaneurysms were classified into two groups as hemorrhagic and exudative in terms of their major clinical sign. Average diameters of the macroaneurysms were arranged in a Gaussian distribution curve (mean and standard deviation: 281.60 +/- 57.28 micrometers). Regarding the distribution curve based on this data macroaneurysm can be defined as being greater than 109.76 micrometers. Macroaneurysms were most frequent on the superotemporal vessels (52.38%), followed by the inferotemporal (38.10%), inferonasal (4.76%) and superonasal vessels. Average diameter (r = +0.68, p = 0.0006) and area (r = +0.71, p = 0.0003) of the macroaneurysms were significantly correlated to the diameter of the relevant arterial segments. The distribution of the macroaneurysms in respect to arterial bifurcation (chi 2 = 18.762, p = 0.0003) and arteriovenous crossings (chi 2 = 8.286, p = 0.0405) were nonrandom with macroaneurysms clustering near this points. Hemorrhagic macroaneurysms were significantly closer to the optic disc (p < 0.01) and were located on relatively larger arterioles (p < 0.01). They were also more circular (p < 0.01) in shape and greater in area (p < 0.01) and diameter (p < 0.01) than the exudative ones. These findings suggest that the location of the macroaneurysm is closely related to its clinical appearance.