Thrombocytosis are conditions frequently encountered in hospitals. This study aims to describe the factors associated with thrombocytosis in hospital patients and out patients. We undertook a case-control study of thrombocytosis between December 01, 2017, and May 31, 2018 in the laboratory of the University Hospital of Befelatanana. Cases are represented by patients who had thrombocytosis in complete blood count. Controls had no thrombocytosis in complete blood count and were matched with cases each day of laboratory analysis. In this study, thrombocytosis decreases with age. Subjects older than 60 years are the least affected (OR, 0.16, 95% CI, 0.05-0.49). Women are less affected than men (OR = 0.89, 95% CI, 0.05-1.59). Thrombocytosis is often discovered accidentally during a health check (OR = 4.78, 95% CI, 1.76-13.21) and frequently affects outpatients. The cases present more abnormalities of blood cells than the controls. These abnormalities are mainly represented by anemia (OR=8.01, 95%CI, 3.55-18.5) and hyperleucocytosis OR=(3.73, 95% CI, 1.69-8.37). Similarly, severe thrombocytosis is significantly associated with leukocytosis (p = 0.02). The blood count should be prescribed for all patients to identify the thrombocytosis that will be treated simultaneously with the disease. Thus, patients will have a good follow-up and their life expectancy will be improved.