The objective of this protocol is to determine if Zenarestat can reverse, stabilize, or slow the progression of diabetic neuropathy relative to placebo and to assess the safety of Zenarestat. Patients with a diagnosis of clinically stable diabetes mellitus with mild distal symmetrical diabetic polyneuropathy will be eligible for this study. This protocol is a double-blind, randomized, placebo-controlled multicenter study consisting of a baseline period followed by a 24-month double-blind treatment period a total of 1350 patients (450 per treatment group) will be randomized. The primary efficacy parameters will be the composite ranked score of nerve conduction velocities (NCV's) for median forearm sensory, peroneal motor, and suran sensory nerves performed in triplicate on the left side unless unavailable. Composite ranked score of quantative sensory testing (QST) for vibratory and cool thermal perception thresholds performed in triplicate on the foot (left side) unless unavailable. The primary analyses will be the composite score of individual NCV's and OST parameters as determined by average ranks, and mean change in NCV for peroneal motor nerve relative to placebo. Each patient's changes relative to baseline will be evaluated using an analysis of covariance with treatment, center and baseline as a covariant. Primary comparisons will be each dose versus placebo using step-down tests of lineartrend. No interim analysis is planned.