In diabetic subjects, polyol pathway activity might inhibit neutrophil function and cause nerve damage. The effects of ponalrestat, an aldose reductase inhibitor, were assessed on neutrophil intracellular killing of Escherichia coli and on autonomic function in diabetic subjects in a randomized double-blind, placebo-controlled, crossover trial. We studied 31 diabetic subjects with autonomic dysfunction and 21 age- and sex-matched control subjects. During two 12-wk treatment periods, the diabetic subjects took either 600 mg of ponalrestat or matching placebo once daily. Neutrophil killing of E. coli was measured by a microbiological assay technique. Kmax by neutrophils from the diabetic subjects was lower than in the control group (Kmax of diabetic subjects 54.5 +/- 26.4 vs. control subjects 67.3 +/- 16.3, P = 0.045). Ponalrestat significantly increased bacterial killing in the diabetic subjects (Kmax of ponalrestat 75.1 +/- 16.5 vs. placebo 58.2 +/- 20.8, P = 0.003) so that there was no longer any significant difference in Kmax between the control subjects and the diabetic subjects on active treatment. Ponalrestat had no significant effect on a range of standard cardiovascular autonomic nerve function tests. We conclude that neutrophil killing of E. coli is impaired in diabetic subjects with autonomic dysfunction. This is restored to normal by ponalrestat.