We have studied the mechanism of origin of 63 cases of trisomy 18. In 2 the additional chromosome was paternal in origin, and in the remaining 61 it was maternal in origin. Both paternal cases were attributable to a postzygotic mitotic (PZM) error. Among the 54 maternal cases for which the cell division of error was established, only 16 were attributable to an error at the first meiotic division (mat MI), whereas no fewer than 35 were due to an error at the second meiotic division (mat MII), the remaining 3 being the result of a PZM error involving the maternal chromosome 18. A standard map of chromosome 18 was constructed and compared with the nondisjunctional map. Approximately one-third of the mat MI errors were associated with complete absence of recombination, whereas in the remaining two-thirds and in all the mat MII errors recombination in the nondisjoined chromosomes appeared to be normal. All the maternal errors were associated with an increased maternal age, although this reached significance only for the mat MII category of nondisjunction. Our observations on chromosome 18 are compared with those on other chromosomes for which there are comparable data.