Objective: The objective of this study was to identify the clinical and pathological factors that can influence the prognosis of breast cancer patients with clinical symptoms of malignant pleural effusion. Methods: This was a clinical cohort study, in which we analyzed the medical charts of patients diagnosed with malignant pleural effusion between 2006 and 2010. By examining the charts, we identified the female patients with a history of breast cancer. For those patients, we collected pathology data related to the primary tumor and cytopathology data related to the pleural metastasis. Results: We evaluated 145 patients, 87 (60%) of whom had tested positive for malignant cells in the pleural fluid. Ductal histology was observed in 119 (82%). The triple-negative breast cancer phenotype was seen in 25 cases (17%). Those patients had the worst prognosis (with a sharp decline in the survival curve), and 20 of the 25 (80%) died during the follow-up period (through June of 2011). The mean survival after the identification of malignant pleural effusion was 6 months. Conclusions: In patients with triple-negative breast cancer who test positive for malignant cells in the pleural fluid, the prognosis is poor and survival is reduced.Keywords: Pleural effusion, malignant/mortality; Breast neoplasms/mortality; Breast neoplasms/genetics. ResumoObjetivo: O objetivo deste estudo foi identificar os fatores clínicos e anatomopatológicos que possam influenciar o prognóstico de pacientes com câncer de mama e sintomas clínicos de derrame pleural neoplásico. Métodos: Trata-se de um estudo clínico de coorte, no qual foram analisados os prontuários médicos de pacientes que receberam diagnóstico de derrame pleural neoplásico entre 2006 e 2010. Por meio da análise dos prontuários, identificamos as pacientes com história de câncer de mama. Para essas pacientes, coletamos dados anatomopatológicos relacionados ao tumor primário e dados citopatológicos relacionados à metástase pleural. Resultados: Das 145 pacientes avaliadas, 87 (60%) apresentaram, no exame citológico, resultado positivo para células neoplásicas no líquido pleural; além disso, 119 (82%) apresentaram tipo histológico ductal. O fenótipo triplo-negativo foi observado em 25 pacientes (17%), as quais apresentaram o pior prognóstico, com queda acentuada na curva de sobrevida. Das 25 pacientes, 20 (80%) evoluíram a óbito durante o período de seguimento (até junho de 2011). A sobrevida média após a identificação de derrame pleural neoplásico foi de 6 meses. Conclusões: Em pacientes com câncer de mama triplo-negativo e exame citológico com resultado positivo para células neoplásicas no líquido pleural, o prognóstico é ruim e a sobrevida é menor. cancer who develop pleural effusion. To that end, we evaluated factors related to the diagnosis, the histological type of the primary tumor, the identification of malignant cells in the pleural fluid, the expression of immunohistochemical markers of breast cancer, and outcomes. MethodsThis was a hospital-based, historical cohort study...