Purpose To determine dose-limiting toxicities, maximum-tolerated dose (MTD), pharmacokinetics, and pharmacodynamics of weekly intravenous temsirolimus, a mammalian target of rapamycin (mTOR) signaling pathway inhibitor, in pediatric patients with recurrent or refractory solid tumors. Patients and Methods Cohorts of three to six patients 1 to 21 years of age with recurrent or refractory solid tumors were treated with a 1-hour intravenous infusion of temsirolimus weekly for 3 weeks per course at one of four dose levels: 10, 25, 75, or 150 mg/m². During the first two courses, pharmacokinetic and pharmacodynamic evaluations (phosphorylation of S6, AKT, and 4EBP1 in peripheral-blood mononuclear cells) were performed. Results Dose-limiting toxicity (grade 3 anorexia) occurred in one of 18 evaluable patients at the 150 mg/m² level, which was determined to be tolerable, and an MTD was not identified. In 13 patients evaluable for response after two courses of therapy, one had complete response (CR; neuroblastoma) and five had stable disease (SD). Four patients (three SDs + one CR) remained on treatment for more than 4 months. The sum of temsirolimus and sirolimus areas under the concentration-time curve was comparable to values in adults. AKT and 4EBP1 phosphorylation were inhibited at all dose levels, particularly after two courses. Conclusion Weekly intravenous temsirolimus is well tolerated in children with recurrent solid tumors, demonstrates antitumor activity, has pharmacokinetics similar to those in adults, and inhibits the mTOR signaling pathway in peripheral-blood mononuclear cells. Further studies are needed to define the optimal dose for use in combination with other antineoplastic agents in pediatric patients.