Melanoma is a rare, but dangerous skin tumor that arises from the melanocytes of the epidermis. According to statistics, 25% of melanomas develop in the presence of dysplastic nevus. Objective — to determine the clinical signs of progressive dysplastic nevus (grades 2—3 and 3 melanocytic dysplasia) that should undergo excisional biopsy in order to prevent melanoma. Material and methods. In 2015 to 2017, excisional biopsy of 25 pigmented masses was performed in patients who had clinically diagnosed dysplastic nevus. The criteria for biopsy were the symptoms of progressive dysplastic nevus, which had been established by the authors in 2013: the changes of the existing nevus in the past 1—5 years or the appearance of a new nevus and its further growth in persons after puberty. The irregular shape of the nevus and its uneven edges and nonuniform color were also taken into account. The mass sizes were 0.15 to 1.5 cm. Results. Fifty-two removed masses showed thin melanoma in 9 (17.3%) cases, a dysplastic nevus in 39 (75%), squamous cell cancer in 1, an intradermal nevus in 1, and keratoma in 2. The 39 dysplastic nevi were found to have grades 1, 2, and 3 melanocytic dysplasia in 11 (28.2%), 19 (48.7%), and 9 (23.1%) cases, respectively. A lentiginous dysplastic nevus was diagnosed in 16 (41%) cases; a mixed dysplastic nevus was detected in 23 (59%). Melanoma developed in the presence of dysplastic nevus in 4 cases and arose in the intact skin de novo in 5. Conclusion. Progressive dysplastic nevi with grades 2—3 and 3 melanocytic dysplasia are to be referred for excisional biopsy. The main symptoms of progressive dysplastic nevus are the change of pre-existing nevus in the past 1—5 years and the emergence of a new nevus and its growth in persons after puberty. Progressive dysplastic nevus and thin melanoma have the same clinical picture. The final diagnosis is established by a histological examination made by an experienced pathologist.