The global incidence of allergic diseases has been on a dramatic increase over the last 20-30 years. This increase is especially pronounced during childhood. It is estimated that 30-40% of people worldwide suffer from one or more allergic diseases, hundreds of millions from allergic rhinitis, and around 300 million have asthma. The prevalence of asthma in adults and children is between 1 and 18% and it is the highest in the English-speaking world. With an 8% incidence of childhood asthma, Croatia belongs to countries with a moderate frequency. In an attempt to explain the epidemiological trends in asthma, theories that defi ne new risk factors for the development and rise of asthma and allergies have emerged. The hygiene hypothesis holds that absent or reduced exposure to certain infections in early childhood can lead to an increased risk of developing allergies. According to epigenetic theory, the increase in allergic diseases is due to environmental exposure (tobacco, traffi c pollution, endotoxins and folates from the diet), either during intrauterine development or early in life, which mediates in gene adaptation to the environment.