Several routinely employed diagnostic methods were analysed for their usefulness in aiding an early and rapid diagnosis of human cytomegalo-virus infection in immunocompromised patients. Clinical samples obtained during an 18-month period were examined by conventional culture, the shell vial method, detection of pp65 antigen and the polymerase chain reaction. Detection of pp65 antigen in peripheral leukocytes was the most useful method for rapid detection of infection at an early stage. Results of other rapid detection methods, the shell vial method and the polymerase chain reaction, gave useful support, while results obtained by conventional culture were not available until after the initiation of therapy. Only a small proportion of serological tests provided useful information for determining whether to treat the patient.