AIMS: To review the diagnostic value of using a thorough necropsy protocol for the investigation of sudden infant deaths, with particular emphasis on the value of routine ancillary laboratory investigations. METHODS: The necropsy and related records of all neonatal, infant, and young childhood deaths (under three years) referred for medicolegal investigation at Dundee from 1990 to early 1998 were reviewed retrospectively. Relevant positive and negative findings were abstracted from the police reports, hospital medical records, necropsy reports, and the results of routine bacteriological, virological, toxicological, and biochemical laboratory investigations. RESULTS: Within the study period, 63 deaths presented as apparent "cot deaths," nine as suspected homicides, nine as neonatal deaths, and 14 in some other manner. An adequate cause of death was identified on the basis of necropsy and laboratory investigations in 35% of the 63 apparent cot deaths, leaving 63% to be finally categorised as sudden infant death syndrome (SIDS). Ten (16%) of the apparent cot deaths were explained on the sole basis of unexpected positive microbiological findings, mostly pneumococcal or meningococcal meningitis and/or septicaemia. Petechial haemorrhages were identified at one or more intrathoracic site in 90% of SIDS and in 55% of explained cot deaths. CONCLUSIONS: Early and extensive laboratory investigations performed routinely in apparent cot deaths provide an unexpectedly high positive diagnostic yield. Routine early bacteriological culture of the CSF (by cisternal puncture) and blood before necropsy should be mandatory in the investigation of all sudden infant deaths.