The many methods used for identifying cytomegalovirus (CMV) infection in immunocompromised patients are discussed. In general, the diagnosis is best made by detecting virus rather than by looking for evidence of an immune response, since the ability of patients to respond immunologically is obviously impaired. Until recently the laboratory took two or three weeks to culture the virus, but more modern methods seem to be capable of providing the same service within 24 hours. These developments will allow infections to be diagnosed sufficiently early for the patients to be entered into trials of potential antiviral agents and for any therapeutic response to be monitored. These modern diagnostic methods are discussed, as is the usefulness of collecting clinical specimens from different anatomical areas as a way of distinguishing symptomatic CMV infection from viral shedding in a patient whose disease is not caused by CMV.