To investigate the relationship between anticoagulation treatment and drug resistance in chest pain, levels of factor Xa residual activity were determined in patients seen in intensive care with recurrent chest pain and compared with levels in patients who had no ischaemic events during hospitalization. A total of 122 patients aged 18 - 75 years who were admitted to hospital with acute coronary syndrome and treated with enoxaparin were included. Of these, 62 patients had recurrent chest pain while hospitalized (group A) and 60 patients had an uneventful follow-up period (group B). Patients requiring primary percutaneous transluminal coronary angioplasty and/or treatment with glycoprotein IIb/IIIa inhibitors, and those with renal failure, a high risk of bleeding or receiving anti-inflammatory drugs were excluded from the study. Median levels (+/- interquartile range) of factor Xa residual activity were significantly higher in group A compared with group B (0.68 +/- 0.29 IU/ml versus 0.34 +/- 0.33 IU/ml). It is concluded that enoxaparin resistance, resulting in high levels of factor Xa residual activity, should be considered in patients with recurrent ischaemia.