Cytomegalovirus (CMV) is often present in bronchoalveolar lavage (BAL) fluid of immunosuppressed patients without CMV pneumonitis. The amount of vital DNA within BAL cells of patients with definite CMV pneumonitis and of vital shedders was quantitated by polymerase chain reaction (PCR) and the extent of CMV gene expression within BAL cells was defined by reverse transcription PCR. No viral DNA was detected in 6 viral shedders, and 12 had low copy numbers (mean, 72 copies/105 BAL cells; median, 20) compared with numbers in pneumonitis patients (267,580 and 57,000, respectively). When CMV intranuclear inclusions were absent within BAL cells of patients with pneumonitis, copy numbers (mean, 9362; median, 7110) were still significantly higher than among shedders. Expression of viral glycoprotein H mRNA was detected in BAL cells of all 11 pneumonitis patients tested but in 0 of 18 viral shedders. Thus, high-grade infection and viral replication within BAL cells are integral features of CMV pneumonitis but not viral shedding.