Rheumatic fever (RF) is an inflammatory disease caused by autoimmune response to a preceding group A streptococcal infection. Mean platelet volume (MPV) reflects the platelet size and the rate of platelet production in bone marrow, and it may be used as an indicator of platelet activation and severity of inflammation. Fifty-three consecutive patients diagnosed with acute rheumatic carditis and 53 control subjects were enrolled into this study. Leukocyte and platelet counts were significantly higher in patients with acute carditis before treatment compared with controls, whereas MPV and platelet distribution width (PDW) values were not significantly different between groups. Platelet counts, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) values were decreased significantly in patients with RF after treatment. There was not a significant difference in terms of platelet count between the controls and the patient group after treatment. ESR was found to be correlated with CRP in patients before and after treatment. In conclusion, the results of our study showed that MPV and PDW levels do not change during acute rheumatic carditis before and after treatment.