We analyzed the medical records of 103 patients with familial adult motor neuron disease (MND). In the 72 families, 329 members were known to be affected. Observations were compared with the sporadic and Mariana forms of MND. Clinical and laboratory examinations of all three forms were similar in clinical course and findings, but there were minor variations in age at onset, sex ratio, survival, and the frequency with which onset occurred in the lower extremities. Recognition of the familial form still depends on diagnosis of the disease in more than one member of a family.