OBJECTIVE: To determine the prevalence of chromosomal abnormalities in female partners of couples undergoing IVF or intracytoplasmic sperm injection (ICSI). DESIGN: Prospective study. SETTING: Leuven University Fertility Center. PATIENT(S): Female candidates for IVF or ICSI. INTERVENTION(S): An initial cytogenetic study was performed on peripheral blood lymphocyte cultures using G- and R-banding. In all patients, > or =25 metaphases were examined. If a chromosomal aberration was detected, additional cytogenetic studies were performed for precise identification. MAIN OUTCOME MEASURE(S): Abnormal female karyotypes in comparison with the general female population. RESULT(S): Cytogenetic analysis was performed in 263 female partners of couples before entering an IVF or ICSI program. The prevalence of autosomal reciprocal balanced translocations was seven times higher in the study group (1.14%) than in the general population (0.16%). All abnormal karyotypes were found in the IVF group with male factor infertility. CONCLUSION(S): Chromosomal abnormalities are more frequent in the female partners of couples seeking fertility treatment. We recommend chromosomal analysis in women before starting IVF or ICSI treatment, even in the presence of male factor infertility.