During a survey of preweaning mortality 538 piglets which died between birth and weaning were autopsied. The results of laboratory examinations permitted a division of the findings into a number of syndromes which were considered to be associated with the immediate cause of death. The non-infectious conditions such as trauma, starvation and suffocation were the most common. Seventy-eight piglets were included in the trauma group because of depressed cranial bone fractures or visceral rupture leading to haemorrhage into the serous cavities. An absence of alimentary tract contents was detected in each of the 92 cases of starvation, fatty metamorphosis of the liver was found in 28 while the hepatocyte cytoplasm of the remainder stained uniformally. Cyanosis and visceral congestion and haemorrhage were the main features observed in the 159 deaths ascribed to suffocation. Bacterial septicaemias and infections with enterotoxic strains of E. coli were the most prevalent infectious conditions. E. coli and beta haemolytic streptococci were the most common causes of septicaemia, being isolated from 36 and 25 cases, respectively. The post-mortem findings in these cases were non-specific except for fibrinous polyserositis which was observed in 11 of the carcasses from which E. coli was recovered. Twenty-six piglets yielded E. coli in heavy pure culture from the upper small intestine and the most common serogroup involved was 08. Encephalomyocarditis virus infection was associated with the deaths of 19 piglets. Pathologically, it was characterised by myocardial degeneration and a variable influx of mononuclear inflammatory cells. No significant lesions were found in 41 piglets.