Objective: psoriasis is a common skin disease in which occurrence rates of clinical features of nail psoriasis vary considerably in the literature. The present study was undertaken to evaluate the frequency of nail involvement in psoriatic patients, to assess the types of nail changes in psoriasis, furthermore to find eventual relationships between nail involvement and some clinical parameters, and finally compare our clinical findings with the few reports available in the literature.Subjects and Methods: From January 2003 to October 2005, patients suffering from psoriasis with nail involvement, attending at a private dermatology clinic, and at the outpatient Clinic of Dermatology in Imam Khoumeini Hospital, Ahwaz, southwest of Iran, were included in the study. The diagnosis was made clinically and in some cases based on histopathological studies.Results: In our study, 500 patients were studied and 296 (59.2%) patients, 177 males (59.8%) and 119 women (40.2%), had nail changes. The mean age of patients was 35±16.95 years and the mean duration of the nail involvement was 6.9±6.88 years. Men involvement percent was a little more than women (59.8% vs 40.2%). The most common clinical type of psoriasis was plaque type psoriasis (67.2%). The most common nail abnormality observed on both finger nails and toe nails was onycholysis (82.8%), followed by pitting (72.6%); subungual hyperkeratosis (72%), nail thickening (37.5%), discoloration of the nail (36.5%), onychodystrophy (12.8%), oil spot (3.7%), Beau's line (7.8%), nail loss (1%), and splinter hemorrhage (0.7%). Of the 296 patients, 227 patients (76.7%) had not involvement of the proximal and lateral nail folds, and 69 (23.7%) had involvement of the proximal and lateral nail folds.Conclusion: The frequency of nail involvement and nail changes in our patients is nearly similar to those in textbooks of dermatology and literatures. About, a third our patients with nail changes, had nail fold involvement, resulting in increased severity of nail changes in these patients.