The purpose of this pilot study was to determine the impact of the length of fixation in 10% buffered formalin on the expression of estrogen receptor by immunohistochemical analysis. We studied tissue samples from 10 invasive breast cancer cases after fixation for 1, 3, 6, and 9 to 10 hours. The tissue was processed immediately after fixation, resembling routine practice. Then the 40 blocks were incubated with antiestrogen receptors SP1, 6F11, and 1D5. The stained slides were reviewed and scored. We found no significant difference in the intensity of the stain or the percentage of cells stained regardless of the time in fixation or the antibody used. Fixation times between 1 and 9 hours in 10% formalin do not seem to have an impact on the expression of estrogen receptor by immunohistochemical analysis, at least in these high-expressing tumors.