Six marrow transplant recipients receiving acyclovir at various dosages for herpesvirus infections developed neurologic symptoms during treatment. Three were receiving concomitant human alpha interferon, and all six had received previous intrathecal methotrexate. Symptoms developed a median of 8 days (range, 2 to 18 days) after initiation of therapy and consisted of lethargy or agitation in five patients, tremor in five, and disorientation or transient hemiparesthesias in one patient each. The only consistent laboratory finding was an abnormal electroencephalogram. Five patients had an increased myelin basic protein level in cerebrospinal fluid. Improvement or resolution of symptoms occurred a median of 13 days (range, 4 to 15 days) after cessation of acyclovir therapy. Acyclovir used at a wide range of dosages may be associated with reversible neurologic symptoms in patients after marrow transplantation. The contribution of previous prophylaxis for central nervous system leukemia, herpesvirus infections, marrow transplantation, or the concomitant use of interferon is unknown.