Three unrelated patients are described, each with a complex,de novo chromosome rearrangement involving four or more break points. One of the patients had few clinical abnormalities and an apparently balanced karyotype with seven break points (1q32, 2q37, 3q26.2, 5q11.2, 5q15, 6q25, 10p13) in six derivative chromosomes. Another patient had multiple congenital anomalies and an apparently balanced complex chromosome rearrangement (CCR) involving four break points (5q13, 5q35, 8p11, 11p15) in three derivatives. The other patient showed multiple anomalies and an unbalanced CCR with seven break points (4q21, 4q25, 6q15, 6q21, 10p13, 10q22, 10q25) in three derivatives including [del(4) (q21q25)]. Each person's parents had normal karyotypes and showed no spontaneous chromosome instability. The fragile sites induced with the FrdU method in two of the three pairs of parents did not correspond to the break points in the CCRs in their offspring. In the last patient, a QFQ-heteromorphism study revealed that del(4) is of paternal origin. The cause of the CCRs in the three patients is unknown. None of their parents had a history of exposure to teratogenic agents or of radiation of the gonads. None of the parents was an atomic bomb victim although four of them lived in Nagasaki.