Mucormycosis is difficult to diagnose. Samples from suspected cases often fail to grow Mucorales in microbiologic cultures. We identified all hematologic malignancy and stem cell transplant patients diagnosed with proven mucormycosis between 2001 and 2009 at Brigham and Women's Hospital/Dana-Farber Cancer Institute. Seminested PCR targeting Mucorales 18S ribosomal DNA and sequencing were performed on formalin-fixed paraffin-embedded tissue samples. Of 29 cases of mucormycosis, 27 had tissue samples available for PCR and sequencing. Mucorales PCR was positive in 22. Among 12 culture-positive cases, 10 were PCR positive and sequencing was concordant with culture results to the genus level in 9. Among 15 culture-negative cases, PCR was positive and sequencing allowed genus identification in 12. Mucorales PCR is useful for confirmation of the diagnosis of mucormycosis and for further characterization of the infection in cases where cultures are negative.