The vascular structure of 5 human malignant melanomas grown in athymic nude mice was characterized. The vessels were filled with a radio-opaque medium administered via the abdominal aorta of the mice. X-ray images, obtained from 720 micrometers-thick tumour sections, provided qualitative information on the vascular structure of the tumours. Histograms for vessel length, surface, and volume as a function of vessel diameter were obtained by stereological analysis of 2 micrometers-thick sections. The volume fraction of necrotic tissue in the tumours was also determined by stereological analysis. The 5 melanomas exhibited individual, characteristic vascular structures as well as individual, characteristic necrotic fractions. The total vessel length ranged from 32 +/- 2 to 80 +/- 4 mm, the total vessel surface from 1.6 +/- 0.1 to 3.8 +/- 0.2 mm2, and the total vessel volume from 0.009 +/- 0.001 to 0.022 +/- 0.002 mm3--all values per mm3 histologically intact tumour tissue. The necrotic fractions ranged from 30 +/- 1 to 49 +/- 4%, and tended to be higher in the poorly than in the well-vascularized melanomas. The volume doubling times ranged from 4.2 to 21.6 days. Melanomas with short volume-doubling times had lower necrotic fractions and tended to be better vascularized than those with long volume-doubling times.