Breast cancer treatment often causes chronic arm oedema. Oedema depends on the balance between microvascular filtration and lymph drainage, but little is known about the microvessels in postmastectomy oedema (PMO). We investigated cutaneous capillary density in PMO, since capillary density is one of the factors influencing fluid load on the lymphatic system. Video-capillaroscopy allows the skin microcirculation to be examined in vivo and recorded for later analysis. Patient arm volumes, measured optoelectronically, increased by 30 +/- 13%. The forearm skin of each arm was examined in 7 normal subjects (mean age 22 years) and 15 PMO patients (mean age 61 years). Native capillaroscopy was found to underestimate capillary density. Venous congestion increased the number of capillaries detected by native capillaroscopy by 26-28%. Fluorescein detected 19-27% more capillaries than native capillaroscopy. Using fluorescein, no significant difference in mean capillary density was found between the right (50 mm-2) and left (48 mm-2) arms of normal subjects (p = 0.53). Surprisingly, cutaneous capillary density in the swollen arm (33 mm-2) was not significantly less than in nonswollen arms (35 mm-2) of patients, despite a mean 14% increase in skin area. Capillary density decreased significantly with age. Relative to the volume and area changes, the density results indicated that cutaneous neovascularization had occurred in the swollen arm.