We retrospectively studied a large cohort of patients with primary antiphospholipid syndrome (APS) from 4 different referral centers to analyze the clinical and serologic features and, specifically, to determine the number of patients going on to develop systemic lupus erythematosus (SLE) or other autoimmune disease after long-term follow-up. The study included 128 unselected patients with primary APS who fulfilled the Sapporo International Criteria from 4 different tertiary hospitals in the United Kingdom, Mexico, and Spain. The patients had attended the referral centers between January 1987 and July 2001. We reviewed clinical and serologic characteristics according to a pre-established protocol. We used univariate analysis with the chi-squared or Fisher exact test and logistic regression to analyze possible factors related to the coexistence of SLE and APS. Ninety-seven female and 31 male patients fulfilled the criteria, with a median age of 42 +/- 12 years (range, 16-79 yr), and with a mean follow-up of 9 +/- 3 years (range, 2-15 yr). The main manifestations included deep vein thrombosis in 62 patients (48%), arterial thrombosis in 63 (49%) patients, pregnancy loss in 177/320 (55%) cases, and pulmonary embolism in 37 (30%) patients. Other clinical manifestations were migraine in 51 (40%) patients, thrombocytopenia in 48 (38%), livedo reticularis in 47 (37%), and valvular disease in 27 (21%). Serologic findings were anticardiolipin antibodies (aCL) IgG positive in 110 (86%) patients, aCL IgM in 36 (39%), lupus anticoagulant in 71 (65%), antinuclear antibodies in 47 (37%), and positive Coombs test in 5 (4%) patients. During the follow-up and after a median disease duration of 8.2 years (range, 1-14 yr), 11 (8%) patients developed SLE, 6 (5%) developed lupus-like disease, and 1 (1%) developed myasthenia gravis. The remaining 110 patients (86%) continued to have primary APS. After the univariate analysis, a family history of lupus, the presence of Raynaud phenomenon, migraine, psychiatric features, multiple sclerosis-like features, hemolytic anemia, low C3 and C4, and Coombs positivity conferred a statistically significant risk for the subsequent development of SLE (p < 0.05). Only the presence of Coombs positivity had statistical significance (odds ratio, 66.4; 95% confidence interval, 1.6-2714; p = 0.027) after the logistic regression evaluation. The current study confirms that progression from primary APS to SLE or lupus-like disease is unusual, even after a long follow-up. Only 3 patients developed anti-dsDNA antibodies. The presence of a positive Coombs test might be a marker for the development of SLE in patients with primary APS.