Background There is limited understanding of how maternal diet affects breastmilk food allergen concentrations, and whether exposure to allergens through this route influences the development of infant oral tolerance or sensitization. Objective To investigate how maternal dietary egg ingestion during early lactation influences egg protein (ovalbumin) levels detected in human breastmilk. Methods In a randomized controlled trial, women were allocated to a dietary group for the first six weeks of lactation: high-egg diet (>4 eggs per week), low-egg diet (one-three eggs per week) or an egg-free diet. Breastmilk samples were collected at 2, 4 and 6 weeks of lactation for the measurement of ovalbumin. The permeability of the mammary epithelium was assessed by measuring the breastmilk sodium : potassium ratio. Egg-specific IgE and IgG4 were measured in infant plasma at 6 weeks, and prior to the introduction of egg in solids at 16 weeks. Results Average maternal egg ingestion was associated with breastmilk ovalbumin concentration. Specifically, for each additional egg ingested per week, there was an average 25% increase in ovalbumin concentration (95% CI: 5-48%, P = 0.01). Breastmilk ovalbumin concentrations were significantly higher in the ` high-egg' group (>4 eggs per week) compared with the ` egg-free' group (P = 0.04). However, one-third of women had no breastmilk ovalbumin detected. No detectable associations were found between mammary epithelium permeability and breastmilk ovalbumin concentrations. Infant plasma egg-specific IgG4 levels were also positively associated with maternal egg ingestion, with an average 22% (95% CI: 3-45%) increase in infant egg-specific IgG4 levels per additional egg consumed per week (P = 0.02). Conclusions and Clinical Relevance Increased maternal egg ingestion is associated with increased breastmilk ovalbumin, and markers of immune tolerance in infants. These results highlight the potential for maternal diet to benefit infant oral tolerance development during lactation.