The therapeutic efficacy of thioctic acid was studied in patients with peripheral diabetic neuropathy. In a double-blind study ten diabetics were treated with thioctic acid or a placebo for 21 days. In a second study ten diabetics were also treated with thioctic acid intravenously (i.v.) for 21 days. Before and on the 11th and 21st day of treatment, we examined the clinical neurological state, the vibration sense according to biothesiometry, the nerve conduction velocity, and the degree of diabetic control. In addition the patients were asked about neuropathic complaints. The therapeutic efficacy of oral or i.v. thioctic acid could not be verified by measurements of the nerve conduction velocity or the vibration sensibility. No effect of oral thioctic acid on subjective complaints was observed. However, i.v. treatment with thioctic acid resulted in a distinct improvement of subjective complaints.