A pericentric inversion of chromosome 12 has been followed in three large independently ascertained Danish families. Out of a total number of 52 persons examined, 25 were found to carry the inversion. The breakpoints in all three families were localized to p13 and q13, resulting in more than one-third of the total length of the chromosomes being inverted. However, no chromosomal aberrations arising because of meiotic crossing-over inside the inverted area have been found among the offspring of the carriers. The percentage of spontaneous abortions among carriers is found to be high, viz. 33%. The segregation rate is calculated to be 0.58, which is not significantly different from an expected segregation rate of 0.5. In family 3, an additional inversion of a chromosome 9 has been found in 4 individuals. Our results are discussed in relation to previous findings and with respect to the genetic counselling of families with pericentric inversions.