Immune complexes have been found in several chronic diseases of unknown aetiology and identification of the constituents of the complexes might lead to recognition of aetiological agents. Sera and renal tissues from patients with amyotrophic lateral sclerosis (A.L.S.) were studied for evidence of immune complexes. C1q precipitation testing demonstrated that sera from 10 of 25 patients with classic A.L.S. bound significantly more radiolabelled C1q than sera from 15 controls. In renal glomeruli studied for deposition of host 1gG, C3, fibrinogen, and albumin by means of direct immunofluorescence, 9 of 33 patients with A.L.S. (27 biopsy and 6 necropsy specimens) had moderate amounts of both IgG and C3 of granular basement membrane and mesangia. This pattern of immunofluorescence is characteristic of immune complex deposits. Of these 9, 8 had rapidly progressive neurological courses, whereas among the remaining 18 patients with no evidence of immune-complex disease, 9 of 12 available for clinical follow-up had stable or slowly progressive courses.