Transmission electron microscopy (EM) is used as an adjunct procedure in the diagnosis of a wide range of neoplastic and non neoplastic diseases and can be extremely useful in well chosen diagnostic dilemmas. The service is regularly inspected and certified by CAP, and is the only diagnostic CAP-certified EM service on NIH campus. A total of 150 cases were processed last year, consisting of 350 blocks. The diagnostic specimens that are submitted to our service for ultrastructural evaluation are from patients treated in NCI/NIH protocols and are diagnostically and scientifically challenging. Due to the nature of the NCI protocols, we are often the first to describe the unusual and unique ultrastructural findings of a tumor or disease, or the outcome of experimental treatments.