A giant storm, the Great White Spot, erupted at the end of September 1990 as a localized, bright cloud system close to Saturn's equator. Its evolution produced a complex planetary disturbance which affected the whole equatorial region a month later. Similar spots have appeared approximately once every saturnian year (about 30 Earth years), implying that a seasonal change - in solar heating, for example - may be responsible for their occurrence. © 1991 Nature Publishing Group.