Impaired platelet function has been associated with an increased propensity for intracerebral hemorrhage (ICH). The role of platelet count and dysfunction in spontaneous ICH (SICH) is poorly understood. We tested the hypotheses that patients with SICH have subtle platelet dysfunction associated with ICH progression and larger ICH size. In a retrospective case series, we compared platelet counts in patients with SICH with age-matched controls with neuromuscular disorders admitted to a Neurosciences Critical Care Unit (NCCU). In a subset of patients, platelet function was measured within one week of ICH. Computerized tomography (CT) scans were performed within 24 hours of the event and ICH volume determined by the ABC/2 method. Comparison of 43 patients with SICH and 35 age-matched controls with neuromuscular disease demonstrated significant decreases in platelet counts over the first few days of admission to the NCCU (Nadir: 149 ± 9 vs 202 ± 12 IU/mm³; P = .001). There was a significant correlation between a fall in platelet count and change in hematoma size in 28 patients (P = .01). Seventeen patients were enrolled prospectively to study platelet function. Patients were divided into 2 groups based on ICH volume: ≤ 30cc and > 30cc. There was an association of low platelet count at a median of 4 days with larger ICH volume (P = .01). Platelet function abnormalities, including aggregation to arachidonic acid, collagen, and ADP and ATP release reactions to thrombin and collagen, and a prolonged bleeding time were common findings in ICH patients compared to standardized controls. Platelet dysfunction was more common in large versus small ICH (80% vs 50%). Two patients with significant (>15%) hematoma enlargement within the first 24 hours had significant early decreases in platelet counts and extensive platelet dysfunction. In conclusion, platelet dysfunction is common among patients with SICH. Low platelet count and platelet dysfunction may be factors in expansion of ICH volume. Further prospective studies with larger sample size are needed to assess this association.