The first example known to us of complementation by two non-homologous chromosomes 3 is present in the karyotype of a phenotypically normal brother of an inv(3)(p25q21) carrier. The normal chromosomes 3 are replaced by two complementary recombinant chromosomes 3. The longer recombinant duplicates 3q21-qter and is deficient for 3p25-pter. It is identical to the recombinant inherited by infants born with multiple congenital anomalies to inv(3)(p25q21) carriers. The shorter recombinant duplicates 3p25-pter and is deficient for 3q21-qter. This recombinant has previously been observed only in prometaphase spreads from sperm of an inv(3) carrier from the same kindred. Theoretically it is possible for the carrier of these complementary recombinant chromosomes 3 to produce sperm carrying either a normal 3, or the inversion 3, which could then fertilize an egg carrying a normal 3 followed by normal fetal development. However, the spouse of our propositus reported one first trimester spontaneous abortion, followed by no recognized pregnancy over the next 12 years of marriage.