Neuroblastomas represent a spectrum of diseases categorized by histological subtypes, age of the patient, and extent of tumor (stage) at diagnosis. In this study we analyzed Ha-ras p21 (protein with molecular weight of 21,000) expression immunohistochemically on 47 primary human neuroblastomas resected at diagnosis. The data demonstrate that the amount of the Ha-ras product correlates with a favorable prognosis (P less than 0.001) and early stages of disease at diagnosis (P less than 0.05). These findings from unmanipulated human neuroblastomas indicate that the Ha-ras gene product (p21) might play a role in the mechanism(s) controlling aggressiveness in this type of tumor in vivo and that the Ha-ras p21 detected by a simple and reproducible immunohistochemical procedure may be of clinical importance in predicting prognosis in patients with this malignancy.