We present herein studies carried out in 17 patients with essential thrombocythemia before treatment with anagrelide and on remission. Ten patients had symptoms related to thrombocythemia, 8 of them at the time of starting treatment. The plasmatic levels of TXB2 and PDGF were measured by ELISA technique. Before treatment, PDGF values corrected for the platelet count were lower than controls, 0.48 ng/10(5) platelets (0.13-1.93) and 0.92 ng/10(5) platelets (0.33-1.16) respectively (p = 0.02), and they were not different from the results obtained during remission. Count-corrected TXB2 levels before treatment were higher than the control group, 1.0 ng/10(5) platelets (0.04-14.4) and 0.25 ng/10(5) platelets (0.13-0.39) respectively (p = 0.04); these values decreased during remission 0.86 ng/10(5) platelets (0.07-9.8) (p = 0.04), although they were still above normal values (p = 0.008). Symptoms disappeared with the normalization of platelet counts in all cases. These results show that patients with essential thrombocythemia during remission have a tendency to normalize the count-corrected TXB2 values.