Studies have demonstrated a familial aggregation of systemic and organ-specific autoimmune diseases. The aim of the present survey was to obtain, by patient interviews, a preliminary estimate of the prevalence of systemic and organ-specific autoimmune diseases among the first- and second-degree relatives of Caucasian patients with connective tissue diseases (CTD) or inflammatory arthritis followed at our unit. Between June 2007 and January 2008, 626 patients and 85 controls (patients with osteoarthritis, osteoporosis, or fibromyalgia) were interviewed. Three hundred ten patients (50%) versus 21 controls (25%) were found to have at least one relative affected with an autoimmune condition (p < 0.0001). The most common conditions were organ-specific autoimmune diseases: 160 (34%) autoimmune thyroid (AT) disease, 112 (24%) psoriasis, 21 vitiligo, and 19 insulin-dependent diabetes mellitus. Systemic autoimmune diseases were reported in 126 relatives: rheumatoid arthritis (66 cases, 14%), 16 sacroileitis, and CTD (43 cases). A significant difference was observed in the prevalence of AT disease between the relatives of the patients and controls (3% versus 0.5%). In conclusion, these data confirm the high prevalence of autoimmune conditions, particularly of AT disease, among the relatives of patients.