1. Flow motion is the cyclical variation in blood flow owing to the rhythmical opening and closing of arterioles. Previous studies have suggested that cutaneous flow motion may be altered in diabetic neuropathy but have not been consistent in their findings. 2. In order to assess the effect of diabetic peripheral neuropathy on flow motion, we have examined the frequency and amplitude of flow motion in 12 patients with diabetic peripheral neuropathy, 10 age-matched diabetic patients without peripheral neuropathy and 10 age-matched non-diabetic controls. 3. Peripheral neuropathy was diagnosed by a history of foot ulceration or chronic painful neuropathy, clinical examination and abnormal peroneal nerve conduction velocities. Blood flow, using laser Doppler flowmetry, was measured at four sites on the dorsum of both hands and feet. Flow motion was analysed using fast Fourier transform analysis, between 0.05 and 0.2 Hz, and displayed on a power spectral density graph. Predominant frequency and relative amplitude of flow motion were calculated. 4. Relative amplitude and frequency of flow motion were similar in the hands of all three groups, as was the frequency in the feet of the three groups. Relative amplitude was significantly smaller in the feet of diabetic patients with neuropathy (median 7.2%, 95% confidence interval 4.9-9.4%) than in diabetic patients without neuropathy (median 13.5%, 95% confidence interval 6.3-21.5%, P < 0.02) or in non-diabetic control subjects (median 10.3%, 95% confidence interval 6.9-27.4%, P < 0.02). 5. Flow motion amplitude is reduced in diabetic peripheral neuropathy. The control of flow motion amplitude appears to be at least partly under neurological control.