SUMMARYWithin a certain North Swedish geographical isolate all marriages between first cousins existing on 1 September 1947 were registered. The data comprise thirty‐four such marriages with a total of 218 live‐born children, and thirty‐two random control families with 165 children. All individuals have been subjected to qualified medical examinations. The following main results were obtained:1The fertility of the cousin marriages did not show any significant deviation from the average fertility in this population.2The mortality of live‐born cousin children was not significantly higher than that of the control children.3The frequency of spontaneous abortions and still‐births was lower in the cousin families. However, the total mortality cannot be claimed to differ significantly from the average in the population.4Cousin parents tend to produce more intellectually defective and less gifted children than average control parents.5The total genetic morbid risk for children from first‐cousin marriages was considerably increased, i.e. about 16% against about 4% for children from random marriages.6A method of estimating the prevalence of detrimental autosomal recessive genes in human populations is outlined. Tentative calculations gave the following results. A minimum of three detrimental autosomal recessive genes in the heterozygous condition per individual is suggested. It seems reasonable to assume that the total number of different autosomal recessive genes (loci) with recognizable major detrimental effect in homozygotes in the population is between 150 and 450.7It is strongly emphasized that the results refer to a specific population. Pending further data from other populations they should not be generalized uncritically. The estimates under (6) are only partly based on precise data, and should therefore only be accepted with great caution.In preparing this paper, it has been a great asset to me to have the privilege of discussing several details with Prof. L. S. Penrose of the Galton Laboratory. I am also grateful for the note he has prepared to supplement