The Nordic medical birth registers have long been used for valuable clinical research. Their collection of data for more than four decades offers unusual possibilities for research across generations. At the same time, serum and blotting paper blood samples have been stored from most neonates. Two large cohorts (approximately 100 000 births) in Denmark and Norway have been described by questionnaires, interviews and collection of biological samples (blood, urine and milk teeth), as well as a systematic prospective follow‐up of the offspring. National patient registers provide information on preceding, underlying and present health problems of the parents and their offspring. Researchers may, with permission from the national authorities, obtain access to individualized or anonymized data from the registers and tissue‐banks. These data allow for multivariate analyses but their usefulness depends on knowledge of the specific registers and biological sample banks and on proper validation of the registers.