Microvessel density (MVD) and area (MVA) were assessed in 53 clinically silent pituitary microadenomas and in surrounding normal pituitary tissue using CD34 immunostaining and digital image analysis system. Twenty-one microadenomas (40%) were avascular; in the others, both MVD and MVA were significantly lower than in the normal pituitary tissue. No significant differences in MVD and MVA were found between hormonally immunonegative and immunopositive tumors or between different hormonal subtypes of immunopositive microadenomas. In microadenomas and in normal pituitary tissue, MVD and MVA were not significantly influenced by age or sex. These results suggest that pituitary adenomas at early developmental stage are either avascular or significantly less vascular than normal pituitary tissue and that differences in vascularity of pituitary adenomas associated with the variables of sex, age, clinical presentation and immunohistochemical phenotype reported in the literature can evolve with tumor growth and possibly acquire statistical significance in large macroadenomas.