Purpose: To determine the recommended phase 2 dose, dose-limiting toxicities (DLT), pharmacokinetic profile, and pharmacodynamics of the heat shock protein (Hsp) 90 inhibitor, 17-allylaminogeldanamycin (17-AAG). Experimental Design: 17-AAG was administered as a 60-min infusion, on days 1, 4, 8, and 11 of a 21-day cycle at dose levels of 150, 200, 270, and 360 mg/m 2/dose. Pharmacokinetic studies and evaluations for Hsp72 and Akt levels in peripheral blood mononuclear cells were done during the first course of therapy. Results: Seventeen patients (7 males), median 7 years of age (range, 1-19 years), were enrolled using a standard dose escalation scheme. No DLTs were observed. Although there were no objective responses, three patients remain on therapy at 6+,7+, and 9+ months with stable disease. One patient with hepatoblastoma had a reduction in α-fetoprotein and stable disease over three cycles. At 270 mg/m2/dose, the Cmax and areas under the plasma concentration-time curves of 17-AAG were 5,303 ± 1,591 ng/mL and 13,656 ± 4,757 ng/mL h, respectively, similar to the exposure in adults. The mean terminal half-life for 17-AAG was 3.24 ± 0.80 h. Induction of Hsp72, a surrogate marker for inhibition of Hsp90, was detected at the 270 mg/m2 dose level. Conclusions:Drug exposures consistent with those required for anticancer activity in preclinical models were achieved without DLT. Evidence for drug-induced modulation of Hsp90 systemically was also detected. The recommended phase II dose of 17-AAG is 360mg/m2/d. Non-DMSO - containing formulations may improve acceptance of this drug by children and their families.