A study of the chromosomes of 14,069 consecutive liveborn infants revealed 11 infants with balanced reciprocal and 13 with balanced Robertsonian translocations. A detailed physical assessment revealed no increase in major or minor malformations over control infants except for an increase in frequency of upslanting palpebral fissures and some possible minor dermatoglyphic changes. A preliminary psychologic assessment of six heterozygotes and 11 controls at 6 years of age revealed no detectable differences. The results of detailed family studies revealed only one child with an unbalanced karyotype, suggesting that where families are ascertained at random through balanced translocation heterozygotes, the frequency of children with unbalanced karyotypes is low in both the t(DqDq) and reciprocal translocation families. Fetal wastage was shown to be slightly increased in reciprocal but not in Robertsonian translocation families. In both the reciprocal and Robertsonian translocation families the segregation ratio for the translocation was not significantly different from 1:1. A significant excess of males was found among reciprocal but not among Robertsonian translocation heterozygotes. A study of the chromosomes involved in 60 reciprocal translocations ascertained at random by conventional staining in the present and two other series suggested that chromosome 11 and perhaps 13 were involved significantly more frequently than might have been expected by chance. However, with the possible exception of band ll(ql3), no “hot” spots for chromosome breakage were observed in these series. Breaks were, however, found to occur significantly more frequently in pale fluorescent, light-staining than in bright fluorescent, dark-staining bands. The importance of these findings in genetic counselling is discus