This paper reports on two male infants with abnormally high levels of serum triacylglycerols (>15.00 mmol/l) and massive accumulation of chylomicrons. Pathological lipidograms were observed during breastfeeding only and were typical of a rare chylomicronaemia syndrome. Laboratory abnormalities were detected accidentally in the otherwise healthy infants. An unrecognized modulating factor in fresh mother's milk caused transitory decreased activity of hydrolytic complex for chylomicrons and very-low-density lipoproteins, probably due to a dysfunction of lipoprotein lipase. The normalization of lipidograms suggested that the catalytic activity of lipoprotein lipase rapidly recovered after weaning. Pathogenesis moved more towards an immune disorder, to the production of (auto)antibodies against a component of the lipolytic system. Mother's milk should be substituted with banked donor human milk, but this is not an unambiguous demand. Conclusion: Fresh mother's milk caused a decrease in the activity of lipolytic enzyme (lipoprotein lipase). The massive hypertriacylglycerolaemia quickly disappeared after weaning. Two infants with this transient chylomicronaemia syndrome were asymptomatic.