Aim: To investigate the relationship between inflammatory markers and platelet size in generalized aggressive periodontitis (GAgP). Material and Methods: Periodontal, inflammatory and platelet indices were compared between 59 GAgP patients and 59 healthy subjects. Gingival biopsies from five patients and five healthy subjects were examined by immunohistochemistry and electron microscopy. Changes in patient periodontal and platelet indices were re-evaluated at 3 months after periodontal therapy. Results: Platelet size was decreased significantly in GAgP patients compared to healthy subjects (p <= 0.003). Weak negative correlations between platelet size and periodontal parameters were found in GAgP patients (p <= 0.025). Platelet aggregates and adhesion to the endothelium or leucocytes were found in venules and connective tissues of gingival biopsies from GAgP patients. Mean platelet volume (MPV) and platelet large cell ratio increased after periodontal therapy in GAgP patients (p <= 0.038). The increase in MPV was related to the decrease in bleeding index in GAgP patients after periodontal therapy (p < 0.001; r = 0.357). Conclusion: Platelet size was reduced in GAgP patients compared to healthy controls, possibly due to the consumption of large platelets at sites of periodontal inflammation. Platelets may be involved in host responses to periodontal infection in GAgP.