Background: Plasma and B cells are implicated in multiple sclerosis (MS) and produce free light chains (FLC) that are excreted in urine. Objective: To confirm that demyelinating diseases (DD) cause increased urinary FLCs. Method: Urinary FLC in 50 patients with DD were compared to 20 patients with posterior uveitis (PU), 19 with AIDS, 34 with rheumatoid arthritis (RA) and 19 normal controls (NC). Result: Subjects with DD, PU, RA and AIDS have higher urinary FLCs than NC (p < 0.01). Urinary FLCs did not correlate with gadolinium-enhancing lesions on MRI. Conclusions: Urinary FLCs are raised in DD. Further studies are required to see if they correlate with disease activity. © 2010 Elsevier B.V. All rights reserved.