In an earlier study of a cohort of 667 patients with systemic lupus erythematosus (SLE) followed for 7.5 months with mean antiphospholipid (aPL) testing 2.5 times, we found a 10% prevalence of definite antiphospholipid syndrome (APS) (2 clinical manifestations and high aPL levels). To determine if more followup and aPL testing increased such prevalence we restudied the cohort after a mean followup of 3.1 years and a mean aPL testing of 5.6 times and found a 15% prevalence of definite APS. Another 21% of patients with SLE had probable APS with either high titers of aPL but only one clinical manifestation or low titers with 2 clinical manifestations. The prevalence of high titer positivity of aPL (IgG and/or IgM isotype) reached 41%. One aPL related feature that increased significantly was livedo reticularis possibly from increased awareness. Factors that influenced significantly the mobility upwards in APS category were more pregnancies and further aPL testing. Conversely, immunosuppressive treatment decreased higher aPL levels. The large number of patients with long disease duration in our cohort showed that the maximum prevalence of definite APS is reached after 15-18 years and is 23%. This might be the highest prevalence of definite APS within SLE.