Background: The possible association between allergy and neoplastic disorders has been the subject of many investigations but no general relationship has been determined. Little attention, however, has been paid to the possible role of allergy in the clinical manifestations of these diseases. In this study, the role of allergy in the susceptibility to uterine leiomyomas and in their growth was investigated. Interaction with ACP < sub>1, a genetic polymorphism associated with the growth of leiomyomas, has been also considered. Methods: Two hundred and three White woman from the population of Rome hospitalized for symptomatic leiomyomas requiring surgical intervention have been studied. One hundred thirty eight healthy women have been considered as controls. Allergy has been evaluated by prick test. T-test for equality of means, analysis of variance and linear correlation analysis has been performed. The level of statistical significance was set at 0.05. Results: The frequency of allergic manifestations in women with leiomyomas does not differ from healthy women. The dimension of leiomyomas is lower in allergic than in non allergic women (p=0.004). The ACP < sub>1 *B/*B genotype and allergy cooperate in lowering the dimension of leiomyomas; the proportion of woman with small leiomyomas (<10 percentile) is much higher in allergic women carrying the *B/*B genotype as compared to other women (p<0.001). About 8% of variance of leiomyomas dimension is attributable to the joint effect of ACP < sub>1 and allergy. Conclusion: Allergic women with high concentration of ACP < sub>1 f isoform (*B/*B genotype) are protected from excessive leyomioma growth. If confirmed in other clinical settings, our observation may have practical importance in identifying women at risk of more severe clinical manifestations.