Patients with rheumatoid arthritis of any degree of severity or duration may develop necrotizing granulomatous pleuritis, a morphologic replica of the inflammatory reaction characteristic of rheumatoid synovitis and rheumatoid nodules. The ensuing pleural effusion may show a unique cytologic picture consisting of slender, elongated multinucleated macrophages, round giant multinucleated macrophages and necrotic background material. A review of 24 such cases from our laboratory as well as the relevant literature clearly demonstrated that not only is this cytologic picture unique, but that it is pathognomonic of rheumatoid disease, even in patients who have not yet developed arthritis.