Objective To study the associations between timing and diversity of introduction of complementary foods during infancy and atopic sensitization in 5-year-old children. Methods In the Finnish DIPP (Type 1 Diabetes Prediction and Prevention) birth cohort (n=3781), data on the timing of infant feeding were collected up to the age of 2years and serum IgE antibodies toward four food and four inhalant allergens measured at the age of 5years. Logistic regression was used for the analyses. Results Median duration of exclusive and total breastfeeding was 1.4 (interquartile range: 0.23.5) and 7.0 (4.011.0) months, respectively. When all the foods were studied together and adjusted for confounders, short duration of breastfeeding decreased the risk of sensitization to birch allergen; introduction of oats <5.1months and barley <5.5months decreased the risk of sensitization to wheat and egg allergens, and oats additionally associated with milk, timothy grass, and birch allergens. Introduction of rye <7.0months decreased the risk of sensitization to birch allergen. Introduction of fish <6months and egg 11months decreased the risk of sensitization to all the specific allergens studied. The introduction of <3 food items at 3months was associated with sensitization to wheat, timothy grass, and birch allergens; the introduction of 12 food items at 4months and 4 food items at 6months was associated with all endpoints, but house dust mite. These results were particularly evident among high-risk children when the results were stratified by atopic history, indicating the potential for reverse causality. Conclusions The introduction of complementary foods was consecutively done, and with respect to the timing of each food, early introduction of complementary foods may protect against atopic sensitization in childhood, particularly among high-risk children. Less food diversity as already at 3months of age may increase the risk of atopic sensitization.