The best way to make the diagnosis of Pneumocystis jiroveci (carinii) pneumonia is to perform a Gomori methenamine silver (GMS) stain on the lung tissue or bronchoalveolar lavage (BAL) fluid. The cyst wall is stained, and the organisms appear as crushed ping-pong balls, or crescent shapes, or folded spheres, or flattened beach balls, or deflated tennis balls. A diagram of this appearance is shown below.