Duplicate platelet survival studies were carried out on 8 patients with giant cell arteritis (GCA), once before the institution of any therapy, and the second time when they were in a completely asymptomatic phase after having received corticosteroid treatment. The time interval between the studies ranged between 5 and 14 months. In the first study the mean peripheral platelet count was 486 +/- 25 X 10(9)/l and in the second 326 +/- 25 X 10(9)/l. The difference between the means was highly significant (P less than 0.001). The mean life-span of the platelets was normal in the duplicate experiments (6.7 +/- 0.3 and 7.3 +/- 0.4 days, respectively). Platelet production rate was significantly (P less than 0.001) raised in the first experiment but became normal in response to corticosteroid therapy. It is concluded that the thrombocytosis seen in GCA is reactive to the inflammation present in this disease, and it seems reasonable to assume that the reduction in the peripheral platelet count which occurs in response to corticosteroid therapy accurately reflects the clinical improvement of the patient.