Reversible nerve conduction block in patients with polyneuropathy after ultrasound thermotherapy at therapeutic dosage. This study investigated the effect of ultrasound on nerve conduction in patients with polyneuropathy. Eight able-bodied controls (Group C) and 16 patients with clinical and physiologic evidence of polyneuropathy were tested. Eight patients (Group NP) had no aching pain symptoms; eight patients (Group P) had severe aching pain, burning sensation, unpleasant tingling, and/or hyperesthesia in the lower extremities. For two minutes, therapeutic ultrasound in doses of 0.5, 1.0, and 1.5W/cm2 were applied over the anterior surface of the leg along the pathway of the deep peroneal nerve. Peroneal nerve conduction studies were performed before, during, and after ultrasound treatment. The compound muscle action potential (CMAP) was recorded from the extensor digitorum brevis muscle. Nerve conduction studies on all eight patients in Group P revealed a significant decrease (41.4% and 44% reduced for doses of 1.0W/cm2 and 1.5W/cm2, respectively; p less than .05) in amplitude of CMAP (from baseline to the first negative peak), and an increase (6.4% and 6.7% increased for doses of 1.0W/cm2 and 1.5W/cm2, respectively; p less than .05) in proximal latency one minute after ultrasound application with a dose of 1.0 or 1.5W/cm2, but not with a dose of 0.5W/cm2 (p greater than 0.1). Changes returned to pretreatment values within five minutes of cessation of ultrasound therapy. In Groups C and NP, there were no significant changes in amplitudes of CMAP or proximal latency before, during, or after ultrasound therapy at a dose of 0.5, 1.0, or 1.5W/cm2. It was concluded that ultrasonic therapy with therapeutic dosage may cause a reversible conduction block on patients with painful polyneuropathy. 