Acute rheumatic fever (ARF) is a systemic inflammatory disease etiologically related to infection with group A streptococcus characterized by a broad spectrum of disorders of cellular and humoral immunity. To estimate the activity of the immunopathological process and to forecast myocardial derangement in ARF patients, measurements were made of neopterin in the serum of ARF patients. Nine men with ARF serving in the armed forces were examined. The control group comprised 24 donors. The reference group included 13 patients with dilated cardiomyopathy and 14 with chronic myocarditis. The mean level of neopterin in ARF patients was equal to 14.5 +/- 12.2 nM/l and was significantly higher than in the donors (5.0 +/- 2.0 nM/l). In patients with dilated cardiomyopathy and chronic myocarditis, it was 9 +/- 6 and 16 +/- 11 nM/l, respectively. On more careful clinical analysis the highest level of neopterin was recorded in 3 patients with impairment of the valvular apparatus of the heart. That level was observable during the whole period of the follow-up of the patients. In other patients, no impairment of the valves was detected, whereas the concentration of neopterin fell to normal. Therefore, the rise of the level of neopterin was described for the first time in patients with ARF. Besides, a relationship was found between the high level of neopterin and impairment of the valvular apparatus of the heart.