In women diagnosed as having category I primary obstetric antiphospholipid syndrome, clinical characteristics and the risk of subsequent thromboembolic events and further unsuccessful pregnancy has not been clearly documented. Women with unexplained obstetric complications and no definite autoimmune systemic diseases were tested for lupus anticoagulant (LA), IgG/IgM anticardiolipin (aCL) and IgG/IgM anti-human beta2-Glycoprotein I (abeta2GPI) antibodies and diagnosed as having primary antiphospholipid syndrome (APS) in classification category I on the basis of more than one laboratory criteria present in any combination. Characteristics at the time of diagnosis and risk factors for subsequent clinical events during a mean follow-up of 6.3 years were evaluated. Fifty-three of 600 women studied were found to fulfil obstetric criteria and had more than one positive laboratory test at the time of diagnosis. All the women were aCL and abeta2GPI positive, and 16 were also LA positive. This latter group (triple positivity) had distinct features and had more frequently experienced previous thromboembolism (OR = 122.5, 95% CI 16-957, p < 0.001). They also had an increased rate of late pregnancy loss (OR = 16.2, 95%CI 0.9-292, p = 0.01), and a higher IgG abeta2GPI titer at diagnosis (median, 25(th) and 75(th) percentile were 118, 37-962, vs. 23, 18-32, respectively, p < 0.0001). During follow-up, the rate of thromboembolic events was significantly higher in the group of women with triple positivity and/ or previous thromboembolism (OR = 57.5, 95% CI 2.7-1160, p = 0.0004) which were the only independent predictors of TE in the multivariate model. Recurrent pregnancy loss took place in seven out of 47 women who had a new pregnancy. Triple positivity and/or previous thromboembolism were again the only independent markers (OR = 34.4, 95% CI 3.5-335.1, p = 0.003) of an unsuccessful new pregnancy. In conclusion, in primary APS with pregnancy morbidity in classification category I, quite different groups of patients may be identified on the basis of laboratory tests. Triple positivity and/or a history of thromboembolism predict new TE events and new unsuccessful pregnancies.