BACKGROUND: Merkel cell (neuroendocrine) carcinoma is a small round blue cell malignant neoplasm that primarily presents in the skin. The diagnosis of Merkel cell carcinoma in a pleural fluid is challenging because of the morphological similarity to many other malignant neoplasms. Immunohistochemical stains can be essential to establish the diagnosis of Merkel cell carcinoma. CASE PRESENTATION: A 77 year-old woman presented with a mass in her right buttock thought clinically to be a boil or sebaceous cyst. Upon histopathologic review including immunohistochemical analysis, a diagnosis of Merkel cell carcinoma was rendered. Wide-excision and sentinel lymph node biopsy revealed negative margins and no evidence of metastasis. Ten months later she complained of bone pain and a bone scan revealed multiple lesions. An abdominal CT scan revealed a T4 vertebral mass and local radiotherapy was administered. Two months later the patient presented with shortness of breath. A chest radiograph showed an effusion and thoracentesis was performed. The fluid was confirmed to contain metastatic Merkel cell carcinoma by cytology and immunohistochemical analysis. CONCLUSIONS: Merkel cell carcinoma is an aggressive neoplasm that can, despite careful surgical management, occasionally present as a malignant pleural effusion in a relatively short time period. Immunohistochemical analysis can aid in confirming this rare outcome.