PURPOSE: This study was performed to detect malignant cells in suspicious cases of malignant pleural effusion by cytogenetic analysis. MATERIALS AND METHODS: Eleven cases with pleural effusion were included in this study. Cells in pleural effusion were treated by direct, or short term, culture to prepare chromosomes. To analyze chromosomes, the G-banding method was used. RESULTS: Chromosome preparations succeeded in 10 cases. 5 cases had normal karyotypes, but in 2 of these cases malignant cells were detected on cytological examination. The other 5 cases had abnormal chromosomes, but on cytological examination showed normal cell appearances. CONCLUSION: Cytogenetic analysis of pleural effusions is not used routinely, but is more sensitive than the cytological examination of malignant pleural effusions. So, chromosome analysis is a good diagnostic tool, when chromosomal abnormalities are detected in an effusion. If a combination of cytology and cytogenetic study are used, the chance of detecting malignant cells in pleural effusion will be higher, and then more invasive diagnostic procedures, such as thoracoscopy or thoracotomy, could be avoided.