Forty-two enucleated eyes of 42 patients with unilateral retinoblastoma were studied histologically, including histochemically examiantion with anti-nm23 polyclonal antibody which does not recognise cDNA but its product. Primary tumours of>15 mm diameter with less evidence of apoptosis and with the most pleomorphic and anaplastic nuclei were associated with an increased risk of distant metastasis, but rosette formation did not discriminate. A high intensity of nm23 staining also indicated a tendency to meastasise, consistent with childhood neuroblastoma but in contrast to findings in carcinoma of the breast, colon and uterine cervix. Copyright (C) 2000 S. Karger AG. Basel.