The clinical response to therapeutic doses of two vitamins were determined in diabetic patients with symptomatic peripheral neuropathy. Of 200 consecutive patients, 100 were randomly allocated to treatment with both thiamine (25 mg/day) and pyridoxine (50 mg/day) group A and the rest group B to treatment with an identical tablet containing 1 mg/day each of thiamine and pyridoxine. Pain, numbness, paraesthesia and impairment of sensation and ankle in the legs were graded into none, mild, moderate or severe. Blood thiamine levels were measured using HPLC fluorimetry. Four weeks after starting treatment the grade was less than on the first visit in 88.9%, 82.5% and 89.7% of those whose worst symptoms were pain, numbness and paraesthesia respectively for group A compared with 11.1%, 40.5% and 39.4% respectively for group B. The severity of signs of peripheral neuropathy decreased in 48.9% of patients in group A compared with 11.4% in group B. The mean (s.e.) pre-treatment whole blood thiamine levels decreased with increasing severity of symptoms: 64.2 (2.81), 57.7 (3.25) and 52.2 (2.14) micrograms/l for those with mild, moderate and severe symptoms respectively (analysis of variance, p = 0.03). Diabetic peripheral neuropathy in Dar es Salaam is associated with thiamine deficiency. Dietary guidelines for diabetic patients should emphasize a balanced diet.