A method is described which has been found capable of detecting subarachnoid hemorrhage (SAH) up to 15 to 17 weeks after its occurrence. The episode of SAH was confirmed by bloody and/or xanthochromic cerebrospinal fluid (CSF) at the time of SAH onset. In this study, 47 samples of lumbar CSF from diagnostically confirmed SAH patients were used. The CSF cells were collected onto slides and stained with May-Gruenwald-Giemsa or Perl's reagent. Iron-positive cells were detected at 1 week, increased by 4 to 6 weeks to 8.5% of total nucleated cells, and decreased to 1% by 15 to 17 weeks. All 27 samples obtained at 2 to 9 weeks after SAH showed iron-positive cells. No iron-positive cells (false-negative samples) were noted in 25% (one of four) of samples obtained during the first week, and in 33% (one of three) of samples obtained 10 to 12 weeks and 15 to 17 weeks after SAH. Of the total samples (37) obtained within 17 weeks after SAH, 8.1% (three of 37) were false negative. No iron-positive cells were detected in samples obtained later than 21 weeks after the SAH episode (10 samples).