The safety of nonsteroidal anti-inflammatory agents in viral infections was assessed in a mouse model of coxsackievirus B4 neonatal myocarditis. Two-day-old mice were infected intraperitoneally with 10(4) TCID50 coxsackievirus B4 and then randomized to receive indomethacin or saline for 7 days. A number of them were killed on designated days. Mortality, viral titers, antibody, and interferon levels plus histopathologic changes in the heart were compared. Among treated animals, mortality was greater (22/45 vs 7/27; p = 0.07) and viral titers were higher on days 4 and 7 (p = 0.038 and 0.028, respectively). Interferon levels were lower on days 4 and 7 (p = 0.028 and 0.008, respectively), and histopathologic changes were more extensive on days 7 and 21 (p = 0.008 and 0.028, respectively). These findings show that indomethacin decreased interferon production, increased coxsackievirus4 titers, and enhanced the virulence of coxsackievirus B4. These results raise significant concerns about the safety of indiscriminate use of nonsteroidal anti-inflammatory agents during severe viral infections.