Allergy is an important risk factor for asthma in children as most of asthmatic children are sensitized. The aim of the study was to investigate the natural course of sensitizations in a group of children (340) at the onset of asthmatic symptoms. We subdivided children in three groups depending on the age at first visit (3-7-11 years) and followed them for 4 years. All children underwent three visits, each including skin prick test, every two years. In the 3-year age group (n = 133), 59% of children were sensitized at first visit, but the percentage increase to 88% at third visit. When looking only at allergic children at first visit (261), we demonstrated that polysensitization became higher than monosensitization since the age of 11. Analysis of sensitization distribution over the time revealed that house dust mites were prominent from the age of 3 years. Parietaria and grass pollens as well as olive pollen significantly increased, particularly during the 4-year follow-up in the 7-year group. Analysis of sensitization score demonstrated that the age range from 3 to 11 is at high risk to polysensitization. In conclusion, this study provides evidence that sensitization is frequent in asthmatic children, polysensitization represents the natural history of allergy and the type of immune response should appear different for each allergen.