This study was undertaken to investigate a possible association of anticardiolipin antibodies (ACLAs) in cancer patients with thromboembolic events. Twenty-five patients with solid tumors complicated with acute thrombosis, 36 cancer patients without any thrombotic events, and a group of 20 healthy volunteers without thrombosis or malignancy were included. The mean age of the cancer patients with and without thrombosis and healthy subjects were 50 years (range 20-75), 45 years (range 23-66), and 40 years (range 20-68), respectively Deep venous thrombosis (n = 16) and thrombosis of the central venous port-catheter systems (n = 9) were confirmed by Doppler sonography in all patients. Ige and IgM isotypes of ACLAs were: quantitated by enzyme-linked immunosorbent assay with normal levels of <23 GPL and <11 MPL, respectively. Mean values of IgG ACLAs were found similar in cancer patients with acute thrombosis (13.8 +/- 4.9 GPL), without thrombosis (12.8 +/- 5.4 GPL) or in healthy subjects (14.8 +/- 5.5 GPL). Although the mean values of IgM ACLAs were within normal limits in all groups, cancer patients with thrombotic events had higher levels of IgM ACLAs (mean = 10.5 +/- 2.2 MPL) than cancer patients without thrombosis (mean = 4.6 +/- 2.3 MPL) (p = .01). Healthy subjects also had lower levels of IgM ACLAs (mean = 7.1 +/- 3.2 MPL) than cancer patients with thrombosis (p = .16). In addition, a higher percentage of cancer patients with or without thrombosis had IgM and IgG ACLA levels above normal limits compared with healthy controls. In conclusion, our study suggests an association between ACLAs or Ige and particularly IgM isotypes and venous thrombosis in malignancy. Identification of cancer patients who are at higher risk for developing thromboembolic events might lead to a better selection of patients for prophylactic anticoagulant therapy.