Objectives The cause of spontaneous subarachnoid haemorrhage (SAH) is not yet fully understood. In recent studies, the role of platelets in SAH has been investigated. However, no study has been found demonstrating the relationship between vasospasm and platelets. This study aimed to investigate the relationship between four indicators of platelet function, platelet count (PC), (10(9)/L) mean platelet volume (MPV) (fL), platelet distribution width (PDW) (%), MPV/PC, SAH and the subsequent vasospasm. Methods This study included 698 patients with cerebral aneurysms who presented with a diagnosis of SAH within 12 h of symptom onset, and a control group of 703 patients with similar demographic characteristics without aneurysms or SAH. Complete blood count tests were performed on blood samples taken from both groups, and PC, MPV, PDW and MPV/PC values were compared statistically. Results While there was no difference between the two groups in terms of PC values, MPV, PDW and MPV/PC values of the patient group were statistically higher than those of the control group (p<0.001, p<0.05 and p<0.001, respectively). In patients who developed vasospasm, MPV value was significantly higher (p<0.01), and PDW value was significantly lower (p<0.01). Conclusions This study found that patients' MPV, PDW and MPV/PC values increase significantly after SAH. Higher MPV and lower PDW were found in patients who developed vasospasm after SAH. These findings suggest that the same mediators released by platelets to stop bleeding in SAH may also be responsible for vasospasm.