Introduction Early infant nutrition is one of the most powerful environmental factors that determine early growth and development. Nutritional factors during a sensitive perinatal period may lead to developmental adaptations proposed to programme for pathological conditions later in life. Among the nutritional factors, essential fatty acids (EFA) and their derivatives play a fundamental role in optimal foetal and postnatal development as structural elements of membranes and as functional modulators. Nevertheless, whether the EFA in general or the n-6 or n-3 EFA speci cally are required for optimal early metabolism and long-term consequences are poorly understood. In animals and humans, variations in the levels of EFA and in the ratio of n-6 to n-3 EFA affect growth, neural development and immune responsiveness of the offspring, and may also have further effects later in life. Leptin is an adipose tissue-derived hormone that regulates food intake and energy expenditure, and is involved in several physiological and pathological processes, including in the perinatal period. During early development, leptin is produced by the placenta and by foetal and neonatal adipose tissues, and is also provided via maternal milk. In humans, circulating leptin levels in early life predict weight gain later in infancy and leptin level is important for normal development and health. Variation in the type of dietary fat during pregnancy and lactation may modulate foetal and neonatal growth and development by leptin-associated mechanisms.