A new syndrome of neonatal convulsions is described. Ninety cases were documented from one maternity hospital between 1973 and 1977. The appearance of this syndrome increased the annual incidence of neonatal convulsions from fewer than 3 to more than 16 per 1000 live births. Fits began on about the fifth day in apparently healthy babies. Investigations and therapeutic trials have not shown the cause. There were no deaths and all babies were discharged from hospital apparently well. The term fifth day fits is suggested with criteria for this diagnosis.