Immunohistochemical staining intensity for ganglioside GD1b was determined for 108 human neuroectodermal tumors. Most of the tissue elements that immunostained were tumor cells; only a few axons and occasional neurons reacted in some specimens. All pilocytic astrocytomas stained very positively, whereas none of the ependymomas and only 11% of primitive neuroectodermal tumors, 20% of glioblastomas, and 28% of anaplastic astrocytomas showed more than faint staining. A similar association between grade and immunostaining was seen in tumors containing an oligodendrogliomatous component, but reactivity was not as strong as in astrocytic tumors or primitive neuroectodermal tumors. Results of Cox regression showed signi cant associations between immunostaining intensity and survival for all cases taken together (P = 0.007); for the group consisting of astrocytomas, oligoastrocytomas, and oligodendrogliomas (P = 0.002); and for astrocytomas alone (P = 0.04). Results were also signi cant using a proportional hazards model controlling for patient age (all cases P = 0.005; astrocytomas only P = 0.02), but not when controlling for tumor grade. Our results indicate that immunohistochemical staining for GD1b is correlated with tumor grade and that it may be of prognostic utility in some primary human brain tumors, especially astrocytomas. Neuro-Oncology 1, 261-267, 1999 (Posted to Neuro-Oncology [serial online], Doc. 98-27, September 9, 1999