Pneumocystis jirovecii pneumonia is a significant cause of morbidity and mortality in AIDS patients as well as those with non-HIV immunosuppressive diseases. To aid diagnosis, the commercial MycAssay Pneumocystis real-time PCR assay (Myconostica, Ltd., Manchester, United Kingdom) targeting the mitochondrial ribosomal large subunit (mtLSU) has been developed to detect P. jirovecii in bronchoalveolar lavage (BAL) specimens. Here, we validated this assay against a laboratory standard of direct immunofluorescence microscopy, a cdc2 real-time PCR assay, or conventional PCR and sequencing of mtLSU. While more sensitive than any of these three assays analyzed individually, the MycAssay Pneumocystis assay demonstrated 100% sensitivity, 100% specificity, a 100% negative predictive value, and a 100% positive predictive value for detecting the presence of P. jirovecii in BAL specimens compared to the laboratory standard. Of note, two samples with positive cycle threshold (C(T)) values according to the MycAssay Pneumocystis assay lacked exponential amplification curves and thus were deemed negative. Also negative according to the laboratory standard, these samples highlight the importance of examining the amplification curves, in addition to noting the C(T) values, when interpreting positive results. Comparison of the MycAssay Pneumocystis assay to a laboratory standard establishes this assay to be a highly sensitive and specific method for the detection of P. jirovecii in bronchoalveolar lavage specimens. The approach may also be useful for the clinical laboratory validation of other sensitive real-time PCR assays.