Asthma and allergies have increased considerably during the past 40-50 years. Along with this increase, a heightened awareness regarding different neuro-developmental disorders such as autism spectrum disorder has occurred and it has been proposed that such disorders are also on the increase. It has been suggested that environmental factors, especially in the indoor environments, may be associated with the increase in these disorder, especially among children, who spend more than 90% of their time indoors. The aim of this thesis has been to investigate certain environmental factors in homes and their impact on children’s health, in terms of asthma, rhinitis, eczema as well as autism spectrum disorders, and to identify certain methodological difficulties in epidemiological investigations. We found that the mean incidence rate per year for doctor diagnosed asthma was in the range of 0.6-2.4% and for incidence of rhinitis 1.1-3.7%. The incidence rate of eczema ever was 2.7%. These results showed that when using a cohort established after birth the estimated incidence rates are strongly dependent of how the baseline population’s health and how the studied health outcome at follow up is defined. Our results showed that the associations between parental reported moisture problems in the home and asthma in children that were revealed in cross-sectional analyses decreased or disappeared when longitudinal data were used on the same data set. Our results therefore indicate that associations between parental reported moisture problems and asthma from cross-sectional questionnaire studies should be interpreted with caution due to the risk for reporting bias. Our results show that children who were living in homes with PVC-flooring in the bedroom in early childhood were more likely to develop asthma during the following 5-year period when compared with children living in homes without such flooring material. A similar association could be seen for children with autism spectrum disorder, where PVC-flooring in early childhood was associated with more reports of autism spectrum disorder five years later. These results indicate that building materials including suspected endocrine disrupting chemicals such as phthalates might be of importance for the development of these chronic diseases. Further studies are needed to explore the early life exposure and the mechanisms and contribution of phthalates for the development of chronic diseases. / Appendix A (en undersökning) och Appendix B (en undersökning)finns i den tryckta versionen av avhandlingen