We have performed a retrospective study to determine if patients with antiphospholipid syndrome that developed systemic lupus erythematosus (APS/SLE) had distinct clinical and/or serological features. All 80 primary APS (PAPS) patients followed up at our APS unit were included in the study and divided into two groups: 14 APS/SLE and 66 PAPS. Prior or at onset of lupus manifestations, six patients were uniformly negative for lupus and Sjögren autoantibodies, and the other eight patients had persistent positive. In the first year after diagnosis of SLE, three patients remained with negative antibodies, the other seven patients maintained the same antibodies, and four patients developed other antibodies. APS/SLE group had a significant lower mean age at PAPS diagnosis (26.0 ± 8.0 vs. 34.2 ± 11.9 years, p = 0.03) and a longer disease duration (14.0 ± 7.0 vs. 6.0 ± 5.0 years, p < 0.0001). The mean time for PAPS to develop SLE was 5.2 ± 4.3 years. The typical clinical and laboratorial findings of APS did not discriminate both groups of patients. At lupus onset, antinuclear antibodies were more frequently observed in those who evolved to SLE (100 vs. 51.5 %, p = 0.0005). Anti-double-stranded DNA (dsDNA), anti-ribosomal P, anti-Ro/SS-A, anti-La/SS-B, and anti-U1RNP antibodies were exclusively found in the APS/SLE patients, whereas anti-Smith (Sm) antibodies were not detected in both groups. The detection of a distinct subgroup of lupus-associated autoantibody in PAPS patients seems to be a hint to overt SLE disease, particularly in those patients with young age at diagnosis.