The drug 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (DHPG) was used to treat serious cytomegalovirus infections in 26 patients with underlying immunodeficiency (including 22 with the acquired immunodeficiency syndrome). In 17 of 22 patients in whom cytomegalovirus was virologically confirmed, clinical status improved or stabilized, although in 4 of them the status of some affected organs deteriorated or did not improve. Fourteen of 18 patients with adequate viral-culture data had clearing of cytomegalovirus from all cultured sites. Patients with cytomegalovirus pneumonia often responded poorly; four of seven died before completing 14 days of DHPG therapy. The condition of 11 of 13 patients with cytomegalovirus retinitis and 5 of 8 with gastrointestinal disease stabilized or improved. However, clinical and virologic relapses occurred in 11 of 14 patients (79 percent) when DHPG was discontinued. Neutropenia was the most frequent adverse reaction. We conclude that DHPG offers promise for the therapy of severe cytomegalovirus infections in some immunodeficient patients, but further study will be necessary to establish its efficacy and safety.