This cross-sectional study evaluated the usefulness of an ultrasound technique in assessment of nail changes in 35 patients with psoriatic onychopathy and 25 with nail dystrophy secondary to onychomycosis. All patients underwent 3 examinations: a complete clinical assessment; a nail ultrasound study; and fungal culture. Nails of patients with psoriatic onychopathy presented a thinner nail plate and nail bed, measured by ultrasound, than did those with onychomycosis. The percentage of patients with a power Doppler signal ≥2 at nail bed was significantly higher in psoriatic onychopathy than in onychomycosis, and structural bone lesions were more frequent in psoriatic onychopathy than in onychomycosis. These results suggest that the presence of structural damage and high-power Doppler signal are the main ultrasound findings supporting a diagnosis of psoriatic onychopathy.