We report two patients with multiple sclerosis which illustrate problems in the interpretation of the cerebrospinal fluid (CSF) protein studies used to confirm diagnosis. In one case a polyclonal increase in IgG synthesis, secondary to an aseptic meningitis, masked oligoclonal banding, which was confirmed on subsequent examination. In the second case, despite a normal quantitative intrathecal IgG synthesis rate, oligoclonal banding in the CSF confirmed abnormal intrathecal synthesis of IgG. These cases show the value of combining a quantitative index of intrathecal IgG synthesis with examination for oligoclonal banding and illustrate their interpretation.