It is believed that malignant cell populations need the development of microvessels to grow and metastasize. The aim of our investigation was to find out whether gamma irradiation can affect proliferation of endothelial cells and thus can affect microvessel density in vivo. We used fertilized chicken eggs. The vascularized part of the yolk sac membrane (area vasculosa) of the eggs received single doses of 2 to 10 Gy. Forty-eight hours after irradiation, the area vasculosa was photographed in vivo, and prints of known magnification were evaluated to determine the density of the blood vessels. Microvessel count is the well-established marker used to determine vascular density. In addition, the proliferative activity of endothelial cells in the yolk sac membrane was determined by estimating the expression of proliferating cell nuclear antigen (PCNA). PCNA immunostaining was assessed immunohistochemically. After a single dose of 10 Gy, a statistically significant increase in vascular density was found compared to values determined at 0, 2, 4 and 8 Gy (P < 0.05). Twenty-four hours after 10 Gy irradiation, 44.8% (mean) of the endothelial cells were PCNA-positive. This was significantly higher (P < 0.05) compared to the results 24 h after 4.0 Gy (22.7%) and compared to control (19.4%). Twenty-four hours later, i.e. 48 h after irradiation with 10 Gy, the endothelial cells also showed a significantly (P < 0.05) higher PCNA positivity with a mean of 34.1% compared to the nonirradiated area vasculosa (18.1%) and compared to the results after 4.0 Gy irradiation (12.0%). The prerequisite for blood vessel formation is the proliferation of endothelial cells. Thus a single-dose irradiation with 10 Gy induces endothelial cell proliferation and subsequent neovascularization in the area vasculosa of the fertilized egg.