An apparent temporal-spatial cluster of Sudden Infant Death Syndrome (SIDS) was noted in the Autonomous Community of Navarre, Spain, when four unrelated children aged between 1 and 6 months died unexpectedly within a 7-day interval in January, 1990. The population under one year of age in Navarre was approximately 4,800. The scan test of temporal clustering showed that the sudden infant deaths occurred closer to one another in time significantly more often than would be expected by chance. All four infants lived in a neighbourhood of the capital of Navarre, which accounts for approximately half the region's population. The clustered cases coincided with an outbreak of influenza type A detected by the epidemiological surveillance system and seen by the increase in 1990 over the same period in the previous year in the number of paediatric emergency-ward admissions during the 'epidemic' days. The results confirm the presence of a temporal-spatial cluster of SIDS and favour an environmental etiology where exposure to influenza A viruses is implicated.