Objective: Episodic volume expansion may complicate the very slow growth pattern of congenital vascular malformations. We investigated the role of microvascular angiogenesis in this process of sudden growth. Method: 100 resection specimens of symptomatic vascular malformations were screened for presence and extent of sheets or clusters immature microvessels, interpreted as microvascular angiogenesis. Microvessel density (MVD), mast cell density (MCD) and Ki67 labelling index of endothelial cells (EC) were assessed immunohistochemically and quantified. Extent of angiogenesis was correlated with the type of vascular malformation and clinical characteristics. Results: Of 107 cases, 71 were arteriovenous malformations (AVM), 21 were venous (VM), and 8 were lymphatic (LM). Microvascular angiogenesis was observed in 30 % of all vascular malformations, of which 94 % appeared to be AVM. MVD, MCD and Ki67 labelling indexes of EC were significantly higher in immature vessel areas. Moreover, in affected patients these angiogenic responses were far more extensive (in terms of area involvement) in men than in women (p<0.05). Conclusion: Microvascular angiogenesis appears a specific feature of the arteriovenous type of vascular malformations, and is much more extensive in men than in women. We suggest that these microvascular responses may contribute to onset of symptoms due to a mass forming effect.