Objective: To assess and compare parameters of pulmonary function in systemic lupus erythematosus (SLE) and anti-phospholipid syndrome (APS) patients. Methods: Consecutive patients (n=74) who were free of respiratory symptoms were divided into four groups: 1) SLE (n=23); 2) SLE with anti-phospholipid antibodies (aPL) (n=18); 3) SLE with APS (n=20); and 4) primary APS (PAPS) (n=13). Pulmonary function testing, single breath diffusion capacity of carbon monoxide (DLCO/SB) and echocardiography studies were performed. Induced sputum cytology was analysed. Results: Forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), and DLCO were significantly reduced in SLE compared to PAPS patients (p=0.039; p=0.017; p=0.029, respectively). Elevated pulmonary arterial pressure was observed in two patients with SLE and aPL and in two with SLE and APS. Lymphocyte and eosinophil counts in induced sputum showed no significant differences; however, a trend towards lower CD4 counts in SLE vs. PAPS was noted (p=0.086), while in patients with both SLE and APS, a low CD4/CD8 ratio was seen. Patients with APS were older than patients without APS (47.12±14.86 vs. 34.29±12.6, p=0.0001), while SLE patients were younger than PAPS patients (38.19±14.68 vs. 48.53±13.97, p=0.023). Conclusion: Abnormal pulmonary functions tests were detected frequently in asymptomatic patients with SLE or PAPS. Although SLE patients were younger, pulmonary function was significantly more impaired in SLE as compared to PAPS patients.