Objective: To investigate the existence of an interchromosal effect in balanced reciprocal translocation carriers and to evaluate their risk of having an affected child or repeated spontaneous abortions.Design: Analysis of chromosomal sperm content by fluorescence in situ hybridization (FISH) using probes for chromosomes 1, 15, 16, 17, 18, X, Y.Setting: University hospital.Patient(s): Six male carriers of balanced chromosome rearrangements, one with normal sperm parameters and five with oligoasthenoterozoospermia, and seven fertile controls.Intervention(s): Fluorescence in situ hybridization for chromosomal enumeration on sperm samples.Main Outcome Measure(s): Rate of disomy for the studied chromosomes.Result(s): A total of 123,842 spermatozoa were scored (82,181 for controls and 41,661 for patients). For each patient, at least one chromosome studied presented a significantly increased rate of disomy. One patient showed a clear interchromosomal effect for at least three of the six chromosomes studied. Disomy rates were statisically significantly and inversely correlated with the total progressive motility of spermatozoa.Conclusion(s): The observed interchromosomal effect seemsto be translocation, patient and chromosome dependant. Variable effects were observed, according to the chromosome studied. When we looked at patients carrying the same translocation, the level of disomy rate was variable and affected differentchromosomes.