Background: Macroaneurysms can represent common consequences of branch retinal vein occlusion(BRVO). The aim of the present study is to evaluate the clinical and angiographic aspects of 31 cases of branch retinal vein occlusions (BRVO) in which retinal macroaneurysms developed, in an attempt to analyze their pathogenic features. Methods: One hundred and sixty-one consecutive patients affected by BRVO were considered. Each patient underwent an ophthalmological examination including fluorescein angiography, at an average interval of two months (range: 1–4 months) from the onset of the disease, with a mean follow-up of 43months (range: 32–56 months). The macroaneurysms were subdivided according to size into small (from 100 to149 microns), medium (from 150 to 249 microns), and large (greater than 250 microns), and according to origin into arterial, venous, capillary and collateral-associated. Results: Thirty-one patients (19.3%) developed retinal macroaneurysms. The total number of detected macroaneurysms was 51; ten (19.6%) were large, 21(41.2%) were of medium-size and 20 (39.2%) were small in dimension. Three lesions were of arterial origin, 22 were capillary and 26 were from collateral vessels. In27 patients (87.1%) the lesions were located outside the macular region, and in 4 patients (12.9%) in the macular region. Patients with retinal macroaneurysms did not show a different prevalence of capillary non-perfusion when compared with others. With regard to the number of retinal venous collaterals patients with macroaneurysms developed fewer than other patients, and the difference was statistically significant (p < 0.001). Conclusion: The insufficient number of retinal venous collaterals can be considered the most contributory factor in the development of macroaneurysms secondary to BRVO.