Acyclovir is known for its antiviral activity against some pathogenic viruses such as the Epstein-Barr virus (EBV) that causes infectious mononucleosis (IM) and IM-like illness. Therefore, we empirically administered acyclovir to patients with suspected EBV-IM and IM like-illness, upon their admission to our hospital. We admitted 25 patients, who were hospitalized for fever and lymphadenopathy, to the Tohoku University Hospital Infectious Disease Ward. As part of treatment, 8 of these patients were given acyclovir (750 mg/day) with their consent and were assigned to the acyclovir group; the remaining 17 patients were assigned to the control group. The mean age of acyclovir patients (all men) was 42±5.2 years, and that of control patients (13 men and 4 women) was 31±3.0 years. The cause of illness was confirmed as EBV-IM in 6 patients (1, acyclovir; 5, control), and remained unknown for the other 19 IM-like illness patients (7, acyclovir; 12, control). A shorter duration of hospitalization and fever was observed in the acyclovir compared to that in the control patients (hospitalization duration: 16±3.7 vs. 27±7.7 days, P=0.36; fever duration: 4.5±1.8 vs. 18±6.5 days, P=0.04). Additionally, serum amyloid A (SAA) levels were lower in acyclovir than that in control patients (98±37 vs. 505±204 µg/mL, P=0.02). Therefore, we propose that acyclovir is a potential therapeutic agent for both EBV-IM and IM like-illnesses. Future studies should further examine its mechanism of action.