Twelve immunocompromised children were treated with 13 courses of intravenously administered ganciclovir for severe cytomegalovirus disease. All children were allograft recipients; 6 received organ transplants (5 liver, 1 kidney) and 6 received bone marrow. They presented with one or more of the following forms of cytomegalovirus disease: pneumonitis, 9; hepatitis, 3; colitis, 2; peritonitis, 1; and retinitis, 1. Clinical improvement was observed in 7 (58%) of 12 patients during ganciclovir therapy. Cessation of active viral replication during therapy accompanied 69% of the treatment courses. Mild and transient increases in creatinine and liver function tests and/or decreases in neutrophil count accompanied 77% of treatment courses but neutropenia (<1000 cells/mm3) did not occur. Transient decreases in platelet counts accompanied therapy in 3 bone marrow allograft recipients, but >50% decrease in lymphocyte count was not seen. We conclude that ganciclovir is safe and appears to have a beneficial effect on cytomegalovirus disease in some pediatric transplant recipients.