Cytomegalovirus has been suggested as a co-factor of disease progression in patients with human immunodeficiency virus type 1 infection. Cytomegalovirus infection is highly prevalent among populations at risk of human immunodeficiency virus 1 infection, and has been associated with both an increased susceptibility to infection and a more rapid course of the disease towards immunodeficiency. Cytomegalovirus can have a direct immunosuppressive effect (through infection of immune cells) and can enhance the replication of human immunodeficiency virus (through the transactivation of the genic immunodeficiency virus expression, the stimulation of cytokine production, and the increase in Fc receptor expression on target cells). The role of cytomegalovirus as a co-factor of the progression towards immunodeficiency in subjects infected with the human immunodeficiency virus type 1 needs to be elucidated with more extensive clinical studies and the application of new molecular biology techniques.