OBJECTIVE immunodepression after stroke is associated with complications like high infection rate, but its role in aneurysmal subarachnoid hemorrhage (aSAH) is unclear. This pilot study aimed to assess the presence of immunodepression and its association with infections after aSAH. METHODS sixteen aSAH patients were enrolled in a prospective study on immune function in a single institution. Detailed immune monitoring (peripheral blood leukocyte subsets, monocyte human leukocyte antigen-DR expression, ex vivo lipopolysaccharide-induced monocytic, Concanavalin A-induced lymphocytic cytokine secretion) was performed until day 10 after aSAH. Occurrence of infection was assessed within 14 days after aSAH. RESULTS sixteen consecutive aSAH patients (53.1 ± 10.2 years; mean ± SD) met the inclusion criteria, classified as asymptomatic (World Federation of Neurological Surgeons; median, 1; quartile, 1-1; n=7) and symptomatic (median, 4; quartile, 3-5; n=9), all presenting with acute neurological deficits, and 5 of these had additional delayed cerebral ischemia. T-lymphopenia, impaired ex vivo lymphocytic/monocytic cytokine secretion, and decreased monocyte human leukocyte antigen-DR expression occurred over all World Federation of Neurological Surgeons grades but persisted beyond day 3 only in symptomatic patients. Pneumonia (67%; P=0.011) was more frequent in symptomatic patients. Already at day 1, patients with pneumonia showed significantly lower T-cell counts and mitogen-induced interferon-γ production compared to patients without infections. CONCLUSIONS a pronounced SAH-induced immunodepression was observed early after aSAH but persisted only in symptomatic patients. Immunodepression was associated with a high incidence of pneumonia. Early diagnosis of immunodepression might allow targeted treatment to prevent infectious complications after aSAH.