The prevalence of circulating antinuclear antibodies (ANA) was studied in a group of 147 patients with multiple sclerosis (MS). Also analyzed was the evolution of ANA over time in those patients for whom the first analysis was positive. ANA titers > or = 1/50 were considered positive. Twenty-two of the MS patients (15%) were positive for ANA upon the first analysis. In a control group of patients with other non-autoimmune neurological diseases the prevalence of ANA positivity was 6.1%. ANA titers became normal in 94.7% of the MS patients over a mean maximum period of 22.5 +/- 19.2 months. We known of no other study of the evolution of ANA titers in MS patients. Because ANA fluctuates in MS, the titers of other circulating autoantibodies should be interpreted cautiously both when discussing etiopathogenetic hypotheses and when diagnosing associated diseases. The diagnostic usefulness of ANA determination in MS is low.