There are many different diseases that can be manifested orassociated with pleural effusion, which may be exudative ortransudative in nature (Nunn & Lumb, 2000).In most instances, the cause of effusion becomes apparent.However, in about 20% of cases, the effusions remain etiologicallyundiagnosed after the initial evaluation (Light, 1998).The DNA content of the cell is a direct reflection of theirposition in the cell cycle. Flow cytometry is used to assay theamount of genetic material in a cell to determine the S-phase fractionor the amount of aneuploidy in a population of cells (Lowell,2001b). Thus DNA cytometry can be performed reliably in thediagnosis of pleural effusions as it made an important contribution tothe confirmation or exclusion of malignancy (Kayser et al, 2000).