Cyclin D1 and cyclin E are overexpressed in approximately 45% and 30% of breast cancers, respectively, and adverse associations with patient outcome have been reported. The potential roles of cyclin D1 and cyclin E expression as markers of therapeutic responsiveness to the pure steroidal antiestrogen ICI 182780 were investigated using T-47D breast cancer cell lines constitutively overexpressing cyclin D1 or cyclin E. Measurement of S phase fraction, phosphorylation states of the retinoblastoma protein, and cyclin E-cyclin-dependent kinase (Cdk) 2 activity demonstrated that overexpression of cyclin D1 decreased sensitivity to antiestrogen inhibition at 24 and 48 h. Overexpression of cyclin E produced a less pronounced early cell cycle effect indicating only partial resistance to antiestrogen inhibition in the short-term. In ICI 182780-treated cyclin D1-overexpressing cells, sufficient Cdk activity was retained to allow retinoblastoma protein phosphorylation and cell proliferation, despite an increase in the association of p21 and p27 with cyclin D1-Cdk4/6 and cyclin E-Cdk2 complexes. After longer-term (>7 days) treatment, antiestrogens inhibited colony growth in cyclin D1- or cyclin E-overexpressing breast cancer cells, but with an approximately 2-2.5-fold decrease in dose sensitivity. This was associated with a fall in cyclin D1 levels, a reduction in the half-life of cyclin D1 protein and a decline in cyclin E-Cdk2 activity in cyclin D1-overexpressing cells, and the maintenance of cyclin E-p27 association in the cyclin E-overexpressing cells. These data confirm that cyclin D1 expression and cyclin E-p27 association play important roles in antiestrogen action, and suggest that cyclin D1 or cyclin E overexpression has subtle effects on antiestrogen sensitivity. Additional studies to elucidate the contribution of alterations in cyclin D1 stability to antiestrogen action and to assess the relationship between antiestrogen sensitivity and expression of cyclin D1, cyclin E, or p27 in a clinical setting are required.