A 5-year retrospective study (January 1981–December 1985) of 87 patients with a known diagnosis of MS who were admitted to the University College Hospital, Galway, Ireland, was carried out. The duration of disease relapse was correlated with the following meteorological factors prevailing about the period of relapse: (a) temperature - mean daily, absolute maximum, absolute minimum; (b) rainfall; (c) relative humidity, and (d) sunshine and global radiation. Highly significant positive correlations were found between relapse duration and temperature variables, total number of sunshine hours and global radiation. Negative correlations were found between relapse duration and both rainfall and relative humidity. Relapses were of longer duration in the middle months of the year.