The importance of immunoglobulin M (IgM) determination in cerebrospinal fluid (CSF) has increased parallel to the need for early diagnosis of infectious and inflammatory disorders of the central nervous system. Current reference values are based on analysis of CSF from 'reference groups' consisting of patients with psychiatric and/or neurological symptoms but without positive clinical findings. Therefore, accurate reference values for CSF IgM are of utmost importance. The present study presents reference values for IgM in CSF in a large sample (n = 111) of healthy individuals with an age span of 18-88 years. The upper reference limit, calculated as the 0.95 fractile, was 0.36 mg/l for CSF IgM, 0.40 for CSF/serum IgM ratio, and 0.045 for the IgM index. These parameters showed no significant difference between sexes nor any significant correlations with age. The correlations between CSF/serum albumin ratio and CSF/S IgM ratio were linear and statistically highly significant, suggesting that the IgM index values do not depend on the blood-CSF barrier function within the normal range of the CSF/serum albumin ratio.