Vascular endothelial cell growth factor (VEGF, a potent endothelial cell mitogen in vitro) may be important in tumour development and its spread in vivo. In this preliminary study, we tested the hypotheses that (i) raised serum levels in Wilms' tumour fall after surgery, and (ii) rising levels predict adverse outcome. Serum VEGF was measured (ELISA) in 13 children about to undergo surgery, and serially on the following day, a week later, and finally 3-6 months after surgery. A simple follow-up at 6 months was also performed. The control group was 60 healthy adults. Before surgery, the median (inter-quartile range) VEGF in the children was 20 ng/ml (10.4-70.5) and was 1 ng/ml (0.5-4.0) in the adults. This difference is statistically highly significant (P=0.0001). After surgery, levels in the children fell significantly to 1.3 ng/ml (0.5-7.95) the following day and to 1.9 ng/ml (0.5-5.0) the following week (P<0.001, ANOVA). Six months after surgery, three of the children had died. A level of >10 ng/ml measured 3 months after surgery correctly identified all three deaths: levels were 1.3 ng/ml (0.5-1.9) in the survivors and were 20.0 ng/ml (15-104) in those who died. These data indicate a rapid reduction in raised VEGF following surgery for Wilms' tumour, but that rising levels predict poor prognosis.