Cytogenetic analyses of 6 unrelated 13–15/21 translocation mongols, 6 unrelated 21–22/21 translocation mongols and almost all of their parents and sibs have been carried out. Parents carrying the translocation chromosome were found in 2 of the 13–15/21 families, but not in any of the 21–22/21 families (one father could not be tested). Mean parental ages for either type of translocation mongol were not significantly different from those for the general population. No translocation mongol had a translocation mongol relative but 4 of the 12 translocation patients had a mongol relative of the standard trisomic type. In an unselected group of 96 mongols 2 were of the translocation type (2.1 %).