Lymphocyte subpopulations from 10 patients who had had a subarachnoid haemorrhage were analysed. In CSF, an increase of CD3 cells was found in two of ten, CD4 in one of ten, CD8 in three of ten, and CD19 in three of ten patients. Three patients with delayed ischaemic deficit (DID) showed a statistically significant increase of intrathecal suppressor/cytotoxic/NK-cells. The cellular inflammation had been modulated within the CNS indicating a pathogenic role in the biochemical cascades following SAH.