An evaluation of 433 children with thrombocytopenia is presented. The material comprises all cases diagnosed in Denmark during the period 1959-1969. The incidence was found to be 31.9 cases per 1,000,000 inhabitants less than or equal to 15 years of age per annum with a peak at the age of 3 to 4 years; the majority of the cases was diagnosed in the spring. Preceding infection, usually associated with fever and localized in the upper respiratory tract, was the most common cause. The majority of the congenital cases was hereditary and associated with other malformations involving in particular the immune system. In about half of the cases the course was spontaneous and no treatment was necessary; the remaining patients were treated with glucocorticoids, including 46 patients, who had undergone splenectomy. 22 patients died, including 16 cases due to serious underlying disorders; in 6 cases the cause of death was haemorrhage (mortality rat=1.4%). At the follow up the platelet count was below 50 mia/1 in 14 patients (4%). No difference was found concerning the platelet count at the follow up between the patients with spontaneous recovery, the patients treated with glucocorticoids and the patients, in whom splenectomy had been performed. During the course the disease autoimmune haemolytic anaemia occurred in 2 patients, whereas no patient developed systemic lupus erythematosus.