BACKGROUND: There is a paucity of longitudinal studies of allergen sensitization in childhood. OBJECTIVE: To investigate the pattern of sensitization in early childhood. METHODS: A nested cohort of children (n = 543) were followed up from birth and given a skin prick test (SPT) at 1, 2, and 3 years of age. A detailed clinical history was obtained. RESULTS: The prevalences of sensitization to aeroallergens were 1.3%, 6.4%, and 10.7% at 1, 2, and 3 years of age. The figures for food allergens were 2.8%, 3.9%, and 3.7%. There was a statistically significant increase in the prevalence of sensitization to >or=1 allergen between years 1 and 2 (P < .001) and years 2 and 3 (P = .032). Among those with a positive SPT at 1 year, 29% tested positive to additional allergens at 2 years (P = .0054). Sensitization to milk or egg at 1 year was a predictor for increased sensitization to peanut at 3 years (odds ratio, 34.8; P < .0001). Sensitization to egg at 1 year was associated with increased sensitization to aeroallergens at 3 years (odds ratios, house dust mite, 27.1, P < .001; cat, 8.9, P < .01; grass, 11.8, P = .005). For peanut and cat allergens, wheal size increases with the age of the child (P = .009 and P = .017, respectively). CONCLUSION: Sensitization to allergens as demonstrated by positive SPT tends to increase with age, and this change can be detected in the first 3 years of life. CLINICAL IMPLICATIONS: The high predictive value for early sensitization and a linear increase in SPT reactivity provide an opportunity for early intervention.