BACKGROUND: Microdialysis has become a routine method for biochemical surveillance of patients in neurosurgical intensive care units. OBJECTIVE: To analyze the intracerebral extracellular levels of 3 interleukins (ILs) during the 7 days after major subarachnoid hemorrhage or traumatic brain injury). METHODS: Microdialysate from 145 severely injured neurosurgical intensive care unit patients (88 with subarachnoid hemorrhage, 57 with traumatic brain injury) was collected every 6 hours for 7 days. The concentrations of IL-1 beta and IL-6 were determined by fluorescence multiplex bead technology, and IL-10 was determined by enzyme-linked immunosorbent assay. RESULTS: Presented are the response patterns of 3 ILs during the first week after 2 different types of major brain injury. These patterns are different for each IL and also differ with respect to the kind of pathological impact. For both IL-1 beta and IL-6, the initial peaks (mean values for all patients at day 2 being 26.9 +/- 4.5 and 4399 +/- 848 pg/mL, respectively) were followed by a gradual decline, with IL-6 values remaining 100-fold higher compared with IL-1 beta. Female patients showed a stronger and more sustained response. The response of IL-10 was different, with mean values less than 23 pg/mL and with no significant variation between any of the postimpact days. For all 3 ILs, the responses were stronger in subarachnoid hemorrhage patients. The study also indicates that under normal conditions, IL-1 beta, IL-6, and IL-10 are present only at very low concentrations or not at all in the extracellular space of the human brain. CONCLUSION: This is the first report presenting in some detail the human cerebral response of IL-1 beta, IL-6, and IL-10 after subarachnoid hemorrhage and traumatic brain injury. The 3 ILs have different reaction patterns, with the response of IL-1 beta and IL-6 being related to the type of cerebral damage sustained, whereas the IL-10 response was less varied.