1. Platelets play a pivotal role during acute ischaemic stroke. An increase in cytosolic Ca(2+) concentrations ([Ca(2+)](i)) triggers intracellular signal transduction, leading to platelet aggregation and thrombosis. In the present study, we examined the differences between platelets from acute ischaemic stroke patients and at-risk controls in terms of the increase in platelet [Ca(2+)](i). 2. Thirty-one patients with acute ischaemic stroke and 27 at-risk controls were enrolled in the present study. Platelet [Ca(2+)](i) was measured using the fluorescent dye fura-2 after stimulation with 100 micromol/L arachidonic acid (AA), 10 micromol/L ADP, 1 micromol/L platelet-activation factor (PAF) and 0.1 U/mL thrombin. 3. Basal [Ca(2+)](i) was higher in the stroke group compared with at-risk controls, irrespective of the presence or absence of extracellular Ca(2+). In Ca(2+)-containing medium, both PAF and ADP, but not AA and thrombin, significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. However, in Ca(2+)-free medium, only PAF significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. Basal [Ca(2+)](i) and PAF-induced platelet [Ca(2+)](i) increases were still higher in the stroke group at the subacute stage than in the at-risk controls. 4. The results of the present study provide direct evidence that Ca(2+) signalling in platelets from acute ischaemic stroke patients was altered in response to particular stimuli. The dysregulation of Ca(2+) movement in platelets may persist up to the subacute stage of ischaemic stroke.