Objective. To evaluate infant and maternal health after assisted reproductive technology (ART), using data on over 90 000 ART children and their mothers in Denmark, Finland, Norway and Sweden, which have been combined and will be compared with a control group of spontaneously conceived children and their mothers. The overall aim of this project is to evaluate the safety of ART. The size of the cohort should enable estimation of the prevalence of rare conditions such as birth defects, cancers, neurological impairments and imprinting diseases in the ART population compared to control children. Outcome data on the mothers of ART children can be used to study risks during pregnancy and obstetric complications after ART. Methods. A personal identification number given to all Nordic residents allows cross-linkage of the national health registers and enables long-term follow-up of ART children. The medical birth registers in the Nordic countries make it possible to cross-link data from mother and child. When a child is identified as conceived by ART, we can obtain a list of all International Classification of Diseases (ICD) codes ever registered on that specific child. Conclusion. Combining the Nordic ART and health registers is a complicated but feasible task. The main strengths of this ongoing study are the size of the cohort of ART children and their mothers and the possibility to follow the children through the health registers. The limitations are related to the national differences in reporting and recording of data together with the heterogeneity of data.