The goal of the UMLS Project is to give practitioners and researchers easy access to machine-readable information from diverse sources. Assessment of the first experimental versions of the UMLS Knowledge Sources is essential to measuring progress toward that goal and to identifying needed enhancements. As of July 30, 1991, copies of the first edition of the UMLS Knowledge Sources had been distributed to 143 individuals and institutions; 66 had provided initial feedback information. The information received indicates that the UMLS Knowledge Sources will undergo broad testing in the patient care, medical education, library service, and product development environments. Preliminary data support the hypothesis that expanded coverage of routine clinical concepts is needed. Key enhancements planned for 1992 and beyond include expanded coverage of ICD-9-CM and CPT.