Corticospinal stimulus conduction was investigated after transcranial magnetic stimulation of the motor cortex in 63 patients (20 female, 43 male, 59 +/- 12 years) with amyotrophic lateral sclerosis (ALS) and progressive bulbar palsy. Recordings were made bilaterally from the Abductor digiti minimi muscle (ADM) in the hand and the Tibialis anterior muscle (TA) in the leg. Thirteen patients were re-examined after 250 +/- 125 days. Eight patients were examined a third time after 552 +/- 165 days. At the first investigation central motor conduction time was abnormal to one or more target muscles in 51% (n = 32) of all patients. No significant delay in CMCT developed during follow-up. The average time of survival of patients with normal CMCT at the first investigation was 16.5 +/- 7.5 months, and 14.7 +/- 8.8 months in patients with abnormal CMCT. This is not a significant difference. It is therefore concluded that transcranial magnetic stimulation is not a sensitive tool in the diagnosis of ALS. Furthermore, CMCT does not provide significant prognostic information.