Background: Allergy is known to be influenced by both diet and the immune system. In addition, the time of first exposure to food allergens and their type appear to play a particularly important role in the development of allergies. Aim: To determine the influence of the time of exposure, and the type, degree of processing, and frequency of supply of complementary foods and the development of sensitization and allergies in children up to three years of age. Materials and metods: The study was conducted prospectively in two stages. The first stage included 106 children aged 6–18 months, while the second stage included 86 children selected from the first stage, after a further 12 months. A questionnaire based on validated FFQ sheets was created for the purpose of the study. The following were assessed: nutrition in the first year of life (time, type, degree of processing), frequency of supply of complementary foods and allergic symptoms, sIgE concentration against 10 foods and 10 inhalant allergens. Four groups of patients were formed. This paper presents the results of the second stage of the study. Results: For all participants, allergenic products, viz. hen’s egg, milk, peanuts, wheat, soybean, fish, tree nuts and shellfish were typically introduced at an age of 7 to 12 months. During this period, egg white was introduced in 47 (85.5%) children with allergy (p= 0.894), in 29 (82.9%) with allergy and sensitization (p= 1.00), and in 38 (82.6%) children with sensitization alone (p= 0.533). Milk was introduced at 7 to 12 months in 35 (64.8%) children with allergy (p= 0.64), 22 (64.7%) with both allergy and sensitization (p= 0.815), and 26 (57.8%) children with sensitization alone (p= 0.627). For other foods, the time of introduction appeared not to significantly influence the presence of allergies or sensitization. Heat-treated peanuts were introduced significantly more often to children without allergies and without sensitization (n= 9; 56.2%) than those without allergies but with sensitization (n= 6; 54.5%) (p= 0.028). Fish was consumed significantly more often by children with allergies, i.e., 1–3x/week (n= 43; 79.6%) than children without allergies, i.e., 1–3x/month (n= 9; 30%) (p= 0.009). Conclusions: No relationship was observed between the introduction time of complementary foods, including allergenic ones, or their type, and the development of allergies and sensitization in children up to three years of age. The degree of processing and the frequency of supply of products may affect the development of allergies and sensitization.