The aim of the study was to determine the progression of muscle weakness in long-term diabetes and its relation to the neuropathic condition. Thirty patients were recruited from a cohort of 92 diabetic patients who participated in a study on muscular function 6-8 years earlier. Nine subjects were nonneuropathic, 9 had asymptomatic neuropathy, and 12 had symptomatic neuropathy. Thirty matched control subjects who participated in the initial studies were also included. At follow-up, isokinetic dynamometry at the ankle, electrophysiological studies, vibratory perception thresholds, and clinical examination (neuropathy symptom score and neurological disability score [NDS]) were repeated. The annual decline of strength at the ankle was 0.7 +/- 1.7% in control subjects, 0.9 +/- 1.9% in nonneuropathic patients, 0.7 +/- 3.1% in asymptomatic neuropathic patients, and 3.2 +/- 2.3% in symptomatic neuropathic patients. In the symptomatic patients, the decline of muscle strength at the ankle was significant when compared with matched control subjects (P = 0.002) and with the other diabetic groups (P = 0.023). Also, the annual decline of muscle strength at the ankle was related to the combined score of all measures of neuropathy (r = -0.42, P = 0.03) and to the NDS (r = -0.52, P = 0.01). In patients with symptomatic diabetic neuropathy, weakness of ankle plantar and dorsal flexors is progressive and related to the severity of neuropathy.