In 3 out of 20 patients with sporadic amyotrophic lateral sclerosis (sALS), cranial magnetic resonance imaging detected multiple demyelinating lesions. All 3 patients died from definite upper and lower motor neuron degeneration. In all 3 cases total cerebro-spinal fluid (CSF) protein remained within normal ranges, and a blood-CSF barrier dysfunction was not detectable. In one of the patients multifocal CNS demyelination coincided with an intrathecal synthesis of immunoglobulin-G and autochthonous CSF oligoclonal IgG banding (OCB) early in disease. Neither absolute or age-corrected survival nor disease progression differed for patients with and without cerebral MR lesions, or normal vs. elevated CSF total protein. Evaluating the CSF in an extended patient sample (n = 29), we found the total CSF protein elevated in 5 of 16 men and none of 13 women (p < 0.05). The mean age-corrected CSF protein content [practical reference limit = (age x 3.3) + 300 mg/l] was higher in male (465 mg/l +/- 32 SE) than in female (350 mg/l +/- 26 SE) sALS patients (p < 0.01). This coincides with a male preponderance in sALS.