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Abstract Background Nail involvement is frequent in patients with psoriasis (Pso) and psoriatic arthritis (PsA) and there is a relationship between nail involvement and inflammation of the enthesis. The main objective of the present study is to describe the ultrasound findings and clinical characteristics of nails from patients with psoriasis and psoriatic arthritis with and without nail dystrophy. Methods A cross-sectional study including consecutive patients with PsO and PsA was carried out. The study patients were divided into 4 groups, totaling 120 participants. Group 1: patients with psoriasis vulgaris and clinically normal nails; Group 2: patients with psoriasis vulgaris and onychodystrophy; Group 3: patients with psoriatic arthritis and clinically normal nails; Group 4: patients with psoriatic arthritis and onychodystrophy; All patients were submitted to dermatological and rheumatological clinical analysis. Ultrasound examinations was performed by a single examiner, blinded to all clinical data, with ultrasound high resolution, in B-mode or gray-scale (GS), Power Doppler (PD) and Spectral Doppler. Results A significant difference was found between the groups regarding the variable Psoriasis Area and Severity Index (PASI) (p = 0.008) and body surface area (BSA) (p = 0.005), with patients with psoriatic arthritis having lower PASI and BSA compared to patients with only cutaneous psoriasis. A positive relationship was found with the average ultrasound thickness of the nail bed and the Nail Psoriasis Severity Index (NAPSI) in correlation analysis (rho = 0.344). When we grouped patients with psoriasis and psoriatic arthritis, there was no significant difference between the cutaneous psoriasis groups and the psoriatic arthritis groups in terms of nail plate GS (p = 0.442), nail bed PD (p = 0.124). Conclusion Greater nail bed thickness indicates early psoriatic nail disease, as confirmed in our study correlating NAPSI with nail bed thickness. Ultrasonography is a low-cost exam, promising in the evaluation, showing that the ultrasound grayscale is consistent with those who have dystrophic nails, but it can’t distinguish psoriasis from psoriatic arthritis, even in those with nail dystrophy.