Objective: Cytospin preparations and immunocytochemistry are common methods in hospitals to evaluate malignancies in body fluids. Characteristics of malignant cells in pediatric body fluids have not been adequately evaluated. Study Design: 183 pleural, peritoneal and pericardial pediatric fluid specimens were examined by cytospin preparations and immunocytochemistry from two hospitals using similar procedural techniques. Cytologic diagnoses were correlated with the results of clinical history, histology and ancillary studies. Results: Forty cases with malignancy were identified (21.9%); the most common diagnoses were rhabdomyosarcoma and acute lymphoblastic lymphoma (9 and 8 cases, respectively). Small round cell tumors revealed similar morphology as clusters of small round cells with central nuclei and scant cytoplasm with frequent small vacuoles. Twenty-one cases were evaluated by immunocytochemistry, 12 by flow cytometry and 5 by cytogenetic analysis. CD3, CD20, TdT, CD10, desmin and myogenin were the most common markers. Staining artifacts causing interpretation difficulties were noted in 5 cases that were resolved by molecular studies and deferral for surgical specimens. Conclusions: Small round cell tumors are the most common malignancies encountered in pediatric body fluids and share a nonspecific morphology. Although immunocytochemistry is helpful to arrive at the correct diagnosis, other ancillary studies may be necessary, particularly in hematologic malignancies and other difficult cases.