Chromosome X numerical abnormalities are frequently observed in non-Hodgkin's lymphoma (NHL), with an incidence of 3% to 14% for chromosomal loss and 7% to 33% for chromosomal gain. Because sex chromosome numerical abnormalities are thought to be due to aging, little information is known about their relation to gender, therapy, and prognosis. Therefore, to determine the incidence and clinical relevance of this abnormality in NHL, we studied specimens from 59 NHL patients (31 men and 28 women) by fluorescence in situ hybridization (FISH) using a directly conjugated centromeric probe for chromosome X. The median age for the entire group was 52 years (range, 31-88 years). All specimens were obtained by fine-needle aspiration of diseased lymph nodes. Sex-matched lymphocytes from benign hyperplastic lymph nodes were used as controls. The overall incidence of chromosome X numerical abnormalities was 49.2%. Female patients had a higher overall incidence than males (76% vs. 24%; p < 0.001). The median percentage of cells involved in this abnormality in each specimen was 5.2%. There was no statistically significant difference in the incidence in previously treated than untreated patients (53.1% vs. 44.4%; p < 0.75) and in intermediate-grade NHL than low-grade NHL (61.1% vs. 50%; p < 0.75). There was a trend towards a higher incidence of chromosome X loss in older patients. While the difference in the incidence of chromosome X abnormalities observed between women and men may be due to the difference in the normal copy numbers of this chromosome in each sex group, this abnormality remained higher than any other autosomal chromosome abnormality in NHL previously evaluated by FISH. We conclude that, although FISH detected a high incidence of chromosome X numerical abnormalities and that females had a higher incidence than males, only a small percentage of the cells were involved, suggesting that this abnormality is most likely a secondary genetic defect that is not important in the pathogenesis of NHL.