Hypermethylation of the calcitonin gene has been described in various hematologic malignancies. In order to assess its frequency and potential usefulness as a marker for leukemic cells and to detect potential clinical correlations, 180 adult patients (aged > 15 years) with newly diagnosed acute leukemia including 133 cases of acute myeloid leukemia (AML) and 47 cases of acute lymphoblastic leukemia (ALL) were tested for its presence in leukemic blasts at diagnosis by Southern blot technique and polymerase chain reaction (PCR) using 3 sets of primers (P550, P566, P1400), amplifying the most frequent sites of hypermethylation upstream or within the gene. In AML, 92 patients (69%) had hypermethylation detected by Southern blot at diagnosis. This hypermethylation could be confirmed by PCR in 18 of 36 tested cases (50%). Hypermethylation was not significantly associated to any clinical or hematological characteristic of the disease. In ALL, 44 patients (94%) had hypermethylation detected by Southern blot at diagnosis. This hypermethylation could be confirmed by PCR in 33 of the 43 tested cases (77%). Sensitivity of PCR assessed by dilution was 1 to 0.1%. Hypermethylation was not either significantly related to any clinical or hematologic characteristics of the disease. Seven ALL cases which were positive by PCR at diagnosis and achieved cytological CR could be tested during CR. Five cases were negative and did not relapse after 3 to 27 months in CR. One case was positive at the beginning of CR and became negative after autologous transplant. However, he relapsed after 9 months in CR, 3 months after the last negative test. PCR for Bcr/Abl was also negative at this time. We conclude that hypermethylation of the calcitonine gene is frequent at diagnosis in adult acute leukemia, particularly in ALL.