The nuclear DNA contents of atypical mesothelial cells from five patients who had an eosinophilic pleural effusion (EPE) were studied by the use of DAPI (4′,6‐diamidino‐2‐phenylindole dihydrochloride) DNA staining. Analysis of the nuclear DNA content revealed a polyploid pattern, with a major peak in the tetraploid region. Using an immunocytochemical technique, the atypical mesothelial cells showed a positive reaction for cytokeratin. In contrast carcinoembryonic antigen (CEA) was always negative in these cells. It is suggested that the atypical mesothelial cells with EPE had a higher rate of proliferation than did the normal mesothelia