The cellular and humoral immunological parameters (leucocyte, granulocyte, lymphocyte, total T, T4, T8 lymphocyte counts, lymphoproliferative response to PHA [LP-PHA), natural killer cell activity [NKCA], IgG, IgM and IgA levels) of 20 pediatric brain tumor patients were investigated before and after chemo-(CT) and radiotherapy (RT) administered according to the UIOI-PBT-91 protocol. The T4 and T8 cell percentages and the LP-PHA values before therapy were found to be significantly diminished in comparison to values obtained from 12 healthy children (p < 0.05). In patients receiving postoperative CT, all cellular immunity parameters except T8 cell number and NKCA; IgG and IgA levels were significantly decreased after two courses of CT (p < 0.05). In 7 patients given postoperative RT, a depression in all cellular immunity parameters was observed (p < 0.05). In 6 patients treated with 2 courses of postoperative CT followed by RT administered concomitantly with low dose CDDP, there was a decrease in all cellular and humoral immunity parameters, which was not found to be significant. In 5/18 patients infectious episodes in mild to moderate severity were observed, none causing mortality. It was concluded that the UIOI-PBT-91 protocol caused cellular immunosuppression both after CT and after RT and some humoral immunosuppression after CT, but was found to be tolerable in regard to acute immunological side effects.