June 2005. Pyothorax-associated lymphoma (PAL) is a rare B-cell non-Hodgkin lymphoma (B-NHL) which develops in the pleural cavity after a 20- to 64-year history of chronic pyothorax. We present here the case of a 62-year-old man who suffered from chronic pyothorax after pneumectomy 44 years ago, and complained of progressive ataxia. A MRI of the head revealed a solitary lesion in the vermis cerebelli, and a biopsy showed a lymphoma displaying immunoblastic features. Immunohistochemistry revealed an aberrant dual B/T phenotype and an Epstein-Barr virus (EBV) type III LMP-1+/EBNA-2+ latency profile. In-situ hybridization disclosed EBV-encoded RNAs in the tumor cells. PCR for the detection of rearranged immunoglobulin heavy chain (IgH) genes followed by GeneScan analysis demonstrated a clonal B-cell population with DNA amplificates of identical size in the brain manifestation, and a large mediastinal tumor analyzed post mortem. Among the largest series of 106 PALs collected through a nationwide survey in Japan, central nervous system (CNS) involvement was detected in 5 (14%) of 36 patients where an autopsy had been performed. To best of our knowledge, this is the first case of a pyothorax-associated lymphoma initially diagnosed on brain biopsy.