The mean area of the foveal avascular zone (FAZ) in normal subjects was reported as 0.231 mm2 to 0.405 mm2, using fluorescein angiography. The FAZ enlarges in vaso-occlusive diseases, especially diabetic retinopathy, sickle cell retinopathy, talc retinopathy and branch retinal vein occlusion. In the present study the FAZ of 20 patients affected by macular branch retinal vein occlusion (MBRVO) was compared with the FAZ of 41 control subjects. The FAZ mean area was 0.56 +/- 0.34 mm2 SD in the MBRVO group, while 0.26 +/- 0.07 mm2 SD in the control group, with a statistically significant difference (p < 0.001). The FAZ mean perimeter was 4.77 +/- 1.90 mm SD in the MBRVO group, and 2.36 +/- 0.32 mm SD in the control group, with a statistically significant difference (p < 0.001). Taking into account the MBRVO group, a statistical correlation was found between visual acuity impairment and FAZ enlargement (p = 0.02), but not between visual acuity impairment and macular edema (p = 0.41). In 14 cases (70%) secondary avascular microzones located in the macular sector correspondent to MBRVO were also evident. MBRVO causes an irregular enlargement of the FAZ which seems to represent the most important feature related to visual acuity impairment.