The clinical and biological features of a series of 27 patients with the recently described primary antiphospholipid syndrome are reported. Most of them belonged to a cohort of 90 patients who were carriers of lupus anticoagulant, which had been detected in the systematic evaluation of prolonged activated partial thromboplastin times in our hospital. Since the diagnosis they underwent a prospective protocol of follow up, with a peak follow up period of 9 years. The mean age of the 27 patients was 40.8 years and there were virtually no differences between sexes. Venous thrombosis was the most common clinical finding (16 episodes in 14 of the 27 patients). The most prevalent laboratory findings were lupus anticoagulant and IgG anticardiolipin antibodies.