The protective effect of increased levels of indirect bilirubin on atherosclerotic heart disease in patients of Gilbert's syndrome is well known. The aim of the study was to investigate the effects of increased levels of bilirubin on the mean platelet volume (MPV) and other hematological parameters. Thirty-two men and 36 women (a total of 68 Gilbert's syndrome patients) and a similar age group of 68 healthy individuals (32 men and 36 women) were included in the study. Hematologic tests, C-reactive protein (CRP) and biochemical values of the two groups were checked. MPV level of Gilbert's syndrome group was 7.8 +/- 1.0fl and CRP 0.2 +/- 0.27mg/dl. In the control group MPV was 8.6 +/- 1.0fl and CRP 0.3 +/- 0.38mg/dl. MPV of patients group (P<0.001) and CRP (P=0.037) were significantly lower than the control group. When dividing Gilbert's syndrome and control groups according to sex into subgroups the level of indirect bilirubin in men with Gilbert's syndrome (1.8 +/- 0.8mg/dl) was found to be higher than other groups. Healthy men had higher levels of MPV (8.8 +/- 0.9fl) whereas Gilbert's syndrome male patients had lower levels (7.7 +/- 1.1fl), (P<0.001). The elevated levels of bilirubin and decreasing levels of MPV and CRP in Gilbert's syndrome patients may have an effect on the slowing down of the atherosclerotic process. (C) 2013 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.