It is unusual to have an opportunity for specific clinical evaluation of patients with infectious disease prior to onset of illness, except in volunteer studies. From 1945 to 1957 sixty cases of acute brucellosis occurred among personnel of a bacteriology laboratory engaged in studies of Brucella melitensis and Brucella suis. These patients had been followed regularly by clinical examination and serological tests before infection occurred. Their known occupational exposure facilitated early diagnosis, hospitalization, and treatment. Many of the infections followed documented laboratory accidents. Most of the patients were observed frequently for one or more years after onset of the acute illness. Analysis of these cases of brucellosis brings out several interesting features of the disease and has made possible appraisal of the incubation period, diagnostic procedures, clinical manifestations, efficacy of chemotherapy, and development of chronic persistent symptoms ("chronic brucellosis"). Clinical Data Each of the 60 patients was examined at the