Eighteen prospectively ascertained cases of brother x sister and father x daughter matings are described. A series of illegitimate children whose mothers were as nearly matched as possible to the incest mothers for intelligence, age, height, weight, and socioeconomic conditions were used as controls. Six of the children of incest had died or were found to have major defects on follow-up 6 months after birth date, whereas one of the comparison children was so classified. This is a larger inbreeding effect than would be predicted on the basis of published findings from marriages of first cousins. The series is published at this time to encourage others to collect these important, but rare and elusive data, in a prospective, controlled manner.