Two unrelated infants with partial trisomy 13 for the distal of the long arm are described. In one, a familial pericentric inversion is present in three generations and crossing-over in the inversion loop is considered as cause of partial trisomy 13. The other showed a tandem duplication of the distal half of the long arm of chromosome 13 beyond 13q14. This is interpreted to have arisen by unequal crossing-over in mispaired synapsis. It is suggested that recombination rather than breaks is a distinctive although rare cause of human chromosomal imbalance.