Estrogen receptor (ER)alpha is a ligand-inducible transcription factor that mediates the physiological effects of 17beta-estradiol (E2). In the uterus, E2 is involved in tissue growth, maintenance, and differentiation. Delta5ERalpha (Delta5) is an ERalpha variant protein expressed in uterine tumors but not in normal tissue. We examined the transcriptional activity of Delta5 and its modulation of human ERalpha basal and E2-stimulated activity in Ishikawa cells, an endometrial cancer cell line. In transient transfection assays, Delta5 increased basal activity of an estrogen response element-containing promoter in the absence or presence of ERalpha but lessened stimulation by ERalpha and E2. Effects of Delta5 were not limited to model reporters, given that cyclin D1 and complement 3 promoters were similarly affected. Increases in basal transcription required dimerization and DNA binding of Delta5, whereas decreased E2 stimulation with ERalpha required only DNA binding. Decreased ligand stimulation was not unique to E2 but also applied to the selective ER modulators tamoxifen and genistein. However, promoter stimulation by epidermal growth factor is retained with Delta5. The ERalpha coactivator small nuclear ring finger protein is expressed in Ishikawa cells and uterine tumors, and it enhances effects of Delta5 alone and with ERalpha on basal activity of an estrogen response element reporter. Thus, in the presence of Delta5 plus ERalpha, there is a lower transcriptional response to E2 and SERMS, but stimulation by epidermal growth factor is retained. The expression of Delta5 in uterine carcinoma may provide a mechanism by which tumors could maintain expression of E2-responsive genes in the absence of E2.