Diagnosing breast carcinoma that has metastasized to body cavity fluids can be difficult. Recently, immunostaining for the facultative glucose transporter Glut-1 has been described as a sensitive and specific means of detecting carcinomas in effusions. However, only five cases of breast carcinoma were studied. We examined Glut-1 specifically as a means of detecting breast carcinoma in effusion cytology. Using avidin-biotin immunocytochemistry, cell block material from 31 cases of breast carcinoma metastatic to body cavity effusions and 33 cases of benign effusions were studied. All cases were immunostained with the Glut-1 antibody. An additional set of slides from these same cases was stained for mucin using the Mayer's mucicarmine technique. Slides were graded for percentage of cells exhibiting immunoreactivity for Glut-1 or for the presence of mucin. Results of staining for both Glut-1 alone and in combination with mucicarmine were compared between the benign and malignant groups. Of the breast cancer cases, 19 of 31 (61%) were immunoreactive for Glut-1, and 25 of 31 (81%) were positive for either Glut-1 or mucicarmine. One of the 33 (3%) benign cases was immunoreactive for Glut-1, and none were positive for mucin. These data suggest that using Glut-1 as a single immunostain or in conjunction with mucicarmine is a specific but modestly sensitive means of detecting breast carcinoma in this cytologic setting.