Forty patients with ALS underwent cortical magnetic stimulation. Twelve had marked pseudobulbar signs; in these motor evoked potentials (MEPs) could not be elicited. Mean MEP latencies in the others, who had predominantly lower motor neuron signs, measured 23.3 +/- 2.1 msec (thenar), 18.7 +/- 5.3 msec (EDC), and 13.4 +/- 2.9 msec (biceps), respectively. These values were significantly longer (P greater than 0.001) compared with normal values (n = 35), which measured 20.2 +/- 1.6, 14.2 +/- 1.7, and 9.4 +/- 1.7 msec, respectively. MEP amplitude was often markedly reduced (less than 15% of the M wave) compared with a normal mean of 39.5 +/- 13.0%. Overall abnormal MEPs (delayed, absent, or reduced in amplitude) approached 100%. It is argued that measuring central motor delay, which was not significantly different in the patients compared with normals, is subject to error in ALS.