Cyclic thrombocytopenia is a rare disease characterized by cyclic oscillations of platelet counts from very low to normal or higher. Severe hemorrhage may occur during the thrombocytopenic phase. To date, treatments for this disorder have been disappointing. Its pathophysiology is unknown. We report a successful outcome using danazol therapy. Prior to danazol treatment, the patient had a 7 year history of cyclic thrombocytopenia, refractory to glucocorticoids, splenectomy, azathioprine, vinca alkaloids, plasma infusions, and hormonal manipulation with Premarin-Provera. Her platelet counts were found to be oscillating in a 21 day cycle between 1 x 10(9)/L and 500 x 10(9)/L. Platelet-associated antibodies were positive and chromium-labeled platelet survival time was shortened. Following 2 months of danazol therapy, her platelet counts at the nadirs were significantly higher than at previous nadirs, and at no time thereafter dropped to the critically low values seen before danazol. Also at 2 months of danazol treatment, the patient reported amelioration of petechiae, and at 9 months it was completely cleared. However, platelet-associated IgG remained positive and platelet counts continued to oscillate, typically between 100 x 10(9)/L and 300 x 10(9)/L in the second year, but stabilized at 3 years, when platelet-associated IgG also disappeared. Danazol was discontinued after 3.5 years. The patient remains in unmaintained remission today, approximately 5 years after discontinuance of danazol. It can be argued that the long-term outcome was due to spontaneous remission. However, significant improvement was noted from the outset of danazol therapy, and further improvement with long-term therapy, as seen in the response of chronic ITP to danazol therapy. Danazol may offer lasting benefit in cyclic thrombocytopenia.