Background: It has been shown that polyphenol-rich foods, such as cocoa, promote health and attenuate, or delay the onset of cardiovascular disease (CVD). The aim of this study was to effect of short-term cocoa supplementation on hemoglobin (Hb) and hematocrit (Hct) after an incremental exhaustive exercise. Materials and methods: Eleven male elite karate-ka were participated in the study. Bruce exercise test was performed as an incremental exhaustive protocol on two occasions, one week apart. In a double-blind design, each participant received either a volume of cocoa solution or placebo (5 mg/kg) before exercise trial. Venous blood samples were collected 2h prior to exercise (base-line), pre, post and 1h after completion of each trial. Differences were examined using Bonferroni test, t-test and two-factor analysis of variance (ANOVA) with repeated measures as appropriate. Results: After the Bruce test, there was significant increase in Hb and Hct levels in both groups (p<0.01). The results showed that in cocoa group, changes of Hb and Hct variables were not significant in all four blood sampling stages. Conclusion: Our results indicated that cocoa did not change the Hb and Hct levels, neither before nor after the exercise. About cocoa and exercise effects on Hb and Hct factors, we need too many researches and investigations (with respect to subjects, exercise protocol, environmental propositions, and cocoa protocol) in future.