This study was designed to explore the prevalence and clinical significance of elevated antiphospholipid antibodies (APA) titres in patients affected by acute myeloid leukemia (AML) and high-grade non-Hodgkin's lymphoma (NHL). We also analyzed possible correlations with circulating levels of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and the soluble form of the receptor for interleukin-2 (sIL-2r). Nineteen patients with de novo AML and 14 patients with newly-diagnosed NHL were investigated. Tests for APA included the measurement of anticardiolipin antibodies (ACA) with a solid-phase immunoassay, and the detection of the lupus-like anticoagulant (LA) activity. Five patients with AML (26.3%) and 5 patients with NHL (35.7%) presented elevated APA at diagnosis, as compared to 3 of 174 persons of the control group (p < 0.0001). APA titres became normal in all patients responding to treatment, whereas non-responders retained elevated levels. In addition, 6 patients (4 with AML and 2 with NHL), who had normal APA at diagnosis and were either refractory to treatment or in relapse, subsequently developed LA and/or ACA positivity. At presentation, the mean levels of IgG- and IgM-ACA in patients were not significantly different from controls, and concordance between ACA and LA results reached just 30%. With regard to the clinical course, we were not able to detect any statistically significant difference between patients with normal and elevated APA. Pretreatment concentrations of IL-6 and TNF-alpha in AML, and sIL-2r in NHL were found significantly elevated compared to controls (p = 0.003, p = 0.009 and p = 0.024 respectively).(ABSTRACT TRUNCATED AT 250 WORDS)