We analyzed the expression of plasminogen activator inhibitor 1 (PAI-1) in 16 leiomyomas and adjacent myometrium of women who underwent a hysterectomy while in the proliferative (n = 8) and secretory phases (n = 8) of the menstrual cycle. We localized the PAI-1 and its mRNA expression in smooth muscle and vessel endothelial cells of uterine tissues using immunocytochemistry and in situ hybridization. The expression of PAI-1 mRNA was higher in 11 (68.75%) of 16 leiomyomas compared with the adjacent myometrium (leiomyoma/myometrium ratio, 1.4-3.0; mean, 2.045). The leiomyoma:myometrium ratio of PAI-1 mRNA expression did not change during the proliferative (Phase I) and secretory (Phase II) phases of the menstrual cycle. In the remaining five samples, the leiomyoma:myometrium ratio of PAI-1 mRNA expression was close to 1 (0.8-1.2; mean, 0.92). Because the locus of the PAI-1 gene is on chromosome 7q22, we screened for loss of heterozygosity (LOH) in these samples using the PAI-1 marker and D7S471, an anonymous marker 12 cM telomeric to PAI-1. Four of five samples with low leiomyoma:myometrium ratio had LOH for the PAI-1 and/or D7S471 markers. The fifth sample demonstrated a noninformative analysis for these markers but had LOH for the D7S515, D7S666, and D7S518 markers, all centromeric to PAI-1. Because del(7)(q22), associated with a relatively low PAI-1 mRNA expression, can deregulate matrix proteinases and growth factors' activity in leiomyomas, it is conceivable that del(7)(q22) results in heterogeneous leiomyoma biology.