In 211 patients with neuroblastoma, serum vanillylmandelic acid (VMA) and homovanillic acid (HVA) levels were determined and correlated to stage, histological differentiation, ferritin, neuron-specific enolase, lactate dehydrogenase (LDH) and outcome. Elevated serum VMA and/or HVA levels were found 16% less frequently than elevated urine levels. The incidence of the elevated serum levels increased with stage (stages I-III 58%, IV 78%, IVS 100%). Increased VMA/HVA ratios were not associated with a higher grade of tumour differentiation. Serum ferritin and neuron-specific enolase showed no correlation, and LDH a borderline non-random correlation with the serum catecholamine metabolites. Using age-related reference values a quotient of serum VMA/HVA (P = 0.061) < 0.7 indicated a poorer event-free survival (48 +/- 10%) than ratios > or = 0.7 (event-free survival 81 +/- 6%) for children with localised neuroblastoma (P = 0.0004). No correlation with prognosis was detected for patients with stage IV and stage IVS disease. We conclude that serum VMA and HVA determinations may be useful as tumour markers for 71% of neuroblastoma patients, and aid in estimating the prognosis in children with localised disease.