To characterize the clinical pattern of psoriatic arthritis (PsA) sine psoriasis.Fifty-seven patients (31 men, 26 women, mean age 46.32 +/- 14.12 yrs) with undifferentiated spondyloarthropathy (SpA) were studied. Two subsets were defined: (1) 21 patients with familial psoriasis (12 men, 9 women, mean age 49.29 +/- 14.17 yrs); (2) 36 patients without familial psoriasis (19 men, 17 women, mean age 44.58 +/- 14.00 yrs). The prevalence of the following clinical variables was evaluated: low back pain, enthesopathy, dactylitis, distal interphalangeal (DIP) arthritis, spinal involvement, and discitis. In all patients the following HLA haplotypes were tested: B7, B13, B17, B18, B27, B38, Cw6, and DR7.Dactylitis and DIP arthritis were markedly present in the articular subset with familial psoriasis (p < 0.0001) that also showed a high frequency rate of HLA-Cw6 (p < 0.0001 vs controls and patients without familial psoriasis). HLA-B27 was markedly frequent in patients without familial psoriasis (p < 0.0001 vs controls and p = 0.019 vs patients with familial psoriasis). In addition, in patients with familial psoriasis the log-linear model showed that the presence of HLA-Cw6 was related to the presence of DIP arthritis as well as dactylitis (likelihood ratio chi-square change of 5.891 and p = 0.015).A subset of patients with PsA "sine psoriasis" is identified by the occurrence of a SpA with dactylitis and/or DIP arthritis, presence of HLA-Cw6, and familial psoriasis in first or second-degree relatives.