Platelet activation at sites of enmeshed sickled red cells in the microcirculation may contribute to platelet plug formation and microinfarction in sickle cell anemia. To test this hypothesis platelets from 116 sickle cell anemia patients free of crisis, 32 patients with crisis, 16 convalescents within 1 week of crisis, and 180 normal controls were studied. Platelets store 90% of their ADP in dense secretory granules. During activation ADP is secreted and permanently lost from the cell. This leads to a decrease in cellular ADP concentration and a sharp rise in the ATP/ADP ratio. ATP and ADP were ethanol-extracted from platelet-rich plasma, measured in the luciferase-luciferin assay and expressed in nmoles per 10(8) cells. No adenine nucleotide differences were found in platelets from patients free of crisis compared with normal controls. The ADP concentration of platelets from patients in crisis was significantly lowered, indicating that in vivo platelet secretion of ADP had occurred. Total and released ADP was decreased from 2.69 to 1.66, and from 1.90 to 1.21 respectively, and the total ATP/ADP ratio was increased from 1.85 to 2.84 (P less than 0.001). ADP stores in platelets from convalescents were significantly different from sickle controls (P less than 0.001) but were less abnormal than ADP stores in platelets from crisis patients (P less than 0.01), indicating recovery. Total and released ADP was decreased to 1.97 and 1.31 respectively, and the ATP/ADP ratio was increased to 2.38. Platelets from patients in crisis were able to release their remaining granular ADP in response to thrombin as effectively as normal platelets. Thus significant platelet activation with ADP release occurs during acute sickle pain crisis. This might contribute to platelet plug formation and microvascular obstruction.