The objective of the present study was to analyse the performance of the tests for detection of anti-beta2 glycoprotein I (beta2 GP I) and anticardiolipin (aCL) antibodies for identification of clinical manifestations of the antiphospholipid syndrome (APS). Patients with systemic lupus erythematosus (SLE) as well as carriers of infectious diseases such as Kala-azar, syphilis and leptospirosis were studied. Particular interest was given to the presence of clinical complications related to APS. Anticardiolipin and anti-beta2 GP I antibodies were searched using an enzyme-linked immunosorbent assay (ELISA) assay. Clinical manifestations of APS were observed in 34 of the 152 patients (22.3%) with SLE and no patient with infectious disease had such manifestations. Antibodies to cardiolipin in moderate or high levels and anti-beta2 GP I were detected in 55 of 152 (36.1%) and 36 of 152 (23.6%) patients with SLE, respectively, and in 2 of 30 (6.6%) and 16 of 30 (53.3%) patients with Kala-azar, in 9 of 39 (23%) and 6 of 34 (17.6%) patients with leptospirosis, and 14 of 74 (18.9%) and 8 of 70 (11.4%) cases of syphilis, respectively. The sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and likelihood ratio (LR) of the anti-beta2 GP I test for the identification of the clinical manifestation of APS were, respectively, 29% [95% confidence interval (CI) = 24%-34%], 78% (95% CI = 73-83%), 15% (95% CI = 11-19%), 89% (95% CI = 85-93%) and 1.38. Regarding the aCL assay, the figure was 29% (95% CI = 24-34%), 76% (95% CI = 71-81%), 14% (95% CI = 10-18%), 89% (95% CI = 86-92%) and 1.26. As the validity and performance of the anti-beta2 GP I assay were similar to the aCL in demonstrating the presence of clinical phenomena associated with APS and due to the difficulties in performing as well as the lack of standardisation of the anti-beta2 GP I test, we suggest that the test for aCL should continue to be the first one performed when the presence of APS is suspected