The perinatal environment plays a crucial role in programming many aspects of adult physiology. Myriad stressors during pregnancy, from maternal immune challenge to nutritional deficiency, can alter long-term body weight set points of the offspring. In light of the increasing concern over body weight issues such as obesity and anorexia in modern societies and accumulating evidence that developmental stressors have long-lasting effects on other aspects of physiology (e.g. fever, pain), we explored the role of immune system activation during neonatal development and its impact on body weight regulation in adulthood. Here we present a thorough evaluation of the effects of immune system activation (LPS; 100 µg/kg i.p.) at postnatal day 3, 7 or 14 on long-term body weight, adiposity, body weight regulation after a further LPS injection (50 µg/kg i.p.) or fasting, and basal and LPS-induced circulating levels of the appetite-regulating pro-inflammatory cytokine, leptin. We show that neonatal exposure to LPS at various times during the neonatal period has no long-term effects on growth, body weight or adiposity. We also observed no effects on body weight regulation in response to a short fasting period, or a further exposure to LPS. Despite reductions in circulating leptin levels in response to LPS during the neonatal period, no long-term effects on leptin were seen. These results convincingly demonstrate that adult body weight and weight regulation are, unlike many other aspects of adult physiology, resistant to programming by a febrile-dose neonatal immune challenge.