Samples were aspirated from 12 thoracic effusions, 10 abdominal effusions and four pericardial effusions in 17 dogs and nine cats. They were analysed cytometrically with the ADVIA 120 flow cytometer and the results were compared with the results of cytological examinations of May-Grünwald-Giemsa-stained smears. The conventional cytology revealed a purulent or pyogranulomatous inflammation in 12 of the animals, lymphoma in six, malignant histiocytosis in two, and an unspecified carcinoma in two; two animals had a chylous effusion, two had a modified transudate, and one dog had an idiopathic pericardial haemorrhage. The flow cytometric analysis was based on cellular volume, peroxidase staining intensity and the determination of nuclear lobularity, and made it possible to identify predominant cell lineages and cell debris, which were shown in characteristic cytograms. Inflammatory effusions, monocytic proliferation and lymphoma were easily detected, but carcinoma cells and mesothelial cells were classified as ‘mononuclear blasts’.