Purpose: To describe pharmacologic variables correlated with the development of neurologic toxicity in patients treated with suramin. Methods: Eighty-one patients were treated with suramin in a phase I study. The rate of drug infusion was continuously adjusted to maintain a preassigned plasma suramin concentration (175, 215, or 275 μg/mL) for a fixed duration (2 to 8 weeks). Results: Eight patients developed grade III/IV neurologic motor impairment (predominantly motor axonal polyneuropathy). All were treated at the 275-μg/mL concentration. One patient treated at the 215-μg/mL concentration developed grade II motor dysfunction. In addition, seven of nine patients had sensory symptoms. Pharmacologic variables associated with the development of polyneuropathy included total cumulative suramin dose, duration of exposure to plasma concentrations greater than 200 μg/mL, and area under the curve (AUC) greater than 200 μg/mL. Conclusion: Significant neurologic toxicity can result from therapy with suramin, even when dosing is designed to avoid exposure to plasma concentrations greater than 350 μg/mL. Future clinical trials of suramin should be designed in such a way as to limit the total cumulative dose to ≤ 157 mg/kg given over a period of ≥ 8 weeks, limit the period of exposure to plasma suramin concentrations greater than 200 μg/mL to ≤ 25 days, and limit the AUC greater than 200 μg/mL to ≤ 48,000 mg · h/L.