Sperm chromosome analysis of 19 sperm donors with either normal or balanced karyotypes was carried out in order to explore the nature of sperm chromosome structural aberrations. A total of 2,389 cells (range 36-298/donor) were karyotyped after in vitro penetration of hamster eggs. The median percentage of sperm structural aberrations was 9.3% (SD +/- 4.7; range 0%-17.8%), with a total of 247 breakpoints, of which 220 could be characterized fully. Two sets of donors were studied in two different centers: center 1 (United States) and center 2 (Spain). The frequencies of nonrejoined and rejoined chromosome-type aberrations were very similar between center 1 and center 2: 83.6% and 10.0%, and 75.0% and 10.3%, respectively. Chromatid-type aberrations were more frequent in center 2 (14.7%) than in center 1 (6.4%) (P = .037). Chromosome 4 had less than the expected number of breakpoints (P < .001). A positive significant correlation was found between sperm breakpoints reported in this study and sites of balanced chromosome de novo rearrangements detected at prenatal diagnosis and reported in the literature (P = .0001).