We studied 200 female patients with multiple sclerosis (MS) to investigate whether pregnancy after the onset of disease influences long term disability. As an index of progression, we used the time between disease onset and wheelchair dependence. Patients who had at least one pregnancy after onset were wheelchair dependent after 18.6 years, versus 12.5 years for the other women (P < 0.0001). This difference remains statistically significant after correction for age at onset of disease.