A new approach, based on the competitive amplification of wild-type and exon-deleted estrogen receptor (ER) variant cDNAs, was used to screen 100 human breast tumors for the presence of ER variants. Already described exon 4-deleted ER mRNA was preferentially detected in tumors with lower grades (P < 0.05) or higher progesterone receptor levels (P < 0.01), whereas new ER variants, deleted in exons 2-4 or in regions within exons 3-7 were associated with higher grades (P < 0.025) and higher ERs (P < 0.001). This approach allows investigation of the expression of multiple ER variant mRNAs and may implicate them as new prognostic markers and as possible contributors to tumor progression.