The karyotypes of 12 fresh breast cancers, including two of the male, selected for their near-diploidy and their slight number of anomalies, ie less than 10 rearranged chromosomes, are reported. A clonal evolution could be demonstrated in 4 cases. Most of chromosomal imbalances result from structural rearrangement, frequently after breakage in juxtacentromeric heterochromatin. There does not seem to exist a specific breakpoint, but many of the imbalances are recurrent. They are, by decreasing order of frequency: gain of 1q,(7 cases), losses of 11q, 16q and 1p (5 cases), losses of 8p and 13q (4 cases), gain of 8q and losses of 6q and 17p (3 cases). None of these anomalies can be regarded as primary, but they are likely to be selected because they confer a slight selective advantage for the carrier cells during the tumoral progression.