The recurrent arteries of Heubner were studied in 30 unfixed human brains (60 hemispheres) obtained from routine autopsies of individuals with a mean age of 34 years. The arteries were injected with tinted polyester resin via cannulation of the internal carotid arteries, and dissected under microscopic magnification. The recurrent artery of Heubner was absent in two hemispheres and double in seven hemispheres, either with a separate origin (in two) or from a common stem (in five). The artery of Heubner had a mean outer diameter of 0.8 +/- 0.04 mm (range 0.3 to 1.5 mm) and a mean length of 23.4 +/- 1.1 mm (range 12 to 38 mm). It originated from the A2 segment of the anterior cerebral artery (ACA) in 57% of the specimens, from the junction of the ACA and the anterior communicating artery in 35%, and from the A1 segment of the ACA in 8%. Three types of recurrent courses were observed. In the Type I or superior course, seen in 41 (63%) of the 65 arteries, the artery followed the superior wall of the A1 segment of the ACA. In the Type II or anterior course, the arteries found in 22 (34%) of specimens maintained a rostral position in relation to the A1 segment. In the Type III or posterior course, taken by two (3%) arteries, a posterior course of the vessel in the anterior perforated substance was found. The branching pattern was identified down to a range of 100 to 200 mu, and the average number of branches was 6.5 +/- 0.4 (range 3 to 12). Four groups of branches were observed. The olfactory group was represented in 91% by a single olfactory branch, with a mean outer diameter of 0.3 +/- 0.03 mm. On average, 1.9 +/- 0.27 frontal branches were found with a mean diameter of 0.13 +/- 0.01 mm. The branches penetrating the anterior perforated substance had a larger outer diameter (mean 0.4 +/- 0.03 mm) with an average number of 2.5 +/- 0.2 branches. The Sylvian fissure branches were more numerous (mean 3 +/- 0.3) with a mean outer diameter of 0.4 +/- 0.03 mm. The point of penetration of the main trunk was found to be constant at the level of the lateral perforated substance-medial Sylvian fissure in 85% of the cases. This report emphasizes the advantages of the intravascular casting resin injection technique in unfixed human brains over other conventional methods. It also describes the application of these anatomic data to the surgical strategy for the anterior circle of Willis, including the possible use of the recurrent artery of Heubner for microvascular reconstructive procedures.