Recently, a G/C polymorphism was found at position -573 of the interleukin-6 ( IL-6) gene promoter. We investigated how this genetic polymorphism relates to IL-6 production and osteoporosis in elderly Japanese women. Genomic DNA was extracted from an aliquot of monocytes in the bone marrow; the monocytes were simultaneously used to form osteoclast-like multinucleated cells (MNCs) and to produce IL-6. Of the 47 subjects with fractures, 96% had a C allele at position -573 of the IL-6 gene. Only 2 subjects possessed homozygotes of G at that position. We investigated IL-6 levels, MNC formation in bone marrow culture, and femoral neck bone mineral density (BMD) in the subjects with the GC and CC genotypes. There were no significant differences between these genotypes as regards IL-6 levels, MNC formation, and femoral neck BMD. However, in the CC genotype, there was a negative relationship between femoral neck BMD and IL-6 levels, and between femoral neck BMD and MNC formation, whereas in the GC genotype and combined (GC + CC) genotypes, femoral neck BMD tended to be related to IL-6 levels and MNC formation. Moreover, the stromal cells in the CC genotype showed higher IL-1alpha-stimulated IL-6 production than did the stromal cells in the GC genotype. Our findings suggest that important information might be obtained not only by continued comparison of different genotypes but also by comparative study within each particular genotype.