Infants with kidney agenesis or dysgenesis, infants with cystic kidneys, and infants with horseshoe kidneys were studied, based on data from three large and population-based congenital malformation registers: a total of 2666 infants among 5.83 million births. There is a strong variability between programs of the rates of registered unilateral kidney malformations and these are strongly over-represented in dead infants or infants with other malformations. There is a male excess but this varies in strength between different types of kidney malformations and between bilateral and unilateral forms. An increased twinning rate was found. The different types of kidney malformation differed with respect to kind of associated non-urological malformations in multimalformed infants. We conclude that for monitoring purposes one should restrict analysis to bilateral forms but that unilateral forms can be of interest in epidemiological analyses.