The present investigations confirm and extend previous reports on the occurrence of cytochemical anomalies among the hemopoietic cells in preleukemia. The cytochemical patterns of the hemopoietic cells obtained from 19 patients suffering from a preleukemic disorder have been studied. The diagnostic value of the cytochemical examination using a panel of 10 different tests is emphasized. In a prospective study in 17 out of 25 patients, preleukemia had been correctly diagnosed. In 4 patients leukemia developed within 2–4 months from the diagnosis (‘imminent leukemia’), in 13 patients leukemia or smouldering leukemia developed between 4 and 25 months after the diagnosis (‘true preleukemia’). The cytochemical profiles of the hemopoietic cells of the latter have been compared with those of 14 patients suffering from aplastic anemia. In preleukemia the blasts did not exceed 3% of the nucleated bone marrow cells. The clinical value of the diagnosis ‘preleukemia’ is discussed as well as the prognosis and the possible therapeut