Gadolinium (III) texaphyrin (Gd-Tex) (NSC 695238) is a potential radiation sensitizer that selectively localizes in tumors and is detectable by magnetic resonance imaging (MRI). In this single-dose phase I trial, reversible renal injury was the dose-limiting toxicity. This report details that renal injury. A single intravenous dose of Gd-Tex was followed 2 hours later by radiation therapy. The Gd-Tex dose was escalated in 13 patient cohorts. Doses ranged from 0.6 to 29.6 mg/kg. The maximum tolerated dosage (MTD) was 22.3 mg/kg. Three patients had grade II and one had grade III acute nonoliguric renal failure at the 22.3 and 29.6 mg/kg dose levels. The injury was always transient, and responded to fluid restriction and renal diet. In all patients, transient green discoloration including urine developed at doses > or =7.1 mg/kg. MRI studies demonstrated image enhancement in the liver, kidneys, and in primary and metastatic tumors in all patients receiving >5.4 mg/kg. It is important that the liver and kidneys be excluded from the radiation volume. Gd-Tex was well tolerated at doses below the MTD. It is important that the liver and kidneys be excluded from the radiation volume. We recommend that 16.7 mg/kg be used as the maximum single dose to obviate even low grade renal toxicity.