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A case of psoriasiform dermatitis in an adult male rhesus macaque is reported. Appearing spontaneously, the condition presented the clinical and histopathological features of human palmoplantar nonpustular psoriasis. The animal developed multiple scaly plaques on his palms and soles, as well as nail hyperkeratosis and widening of the nail root. Microscopically, the skin lesions showed epidermal hyperkeratosis with multifocal parakeratosis, neutrophil microabscesses in the stratum corneum, a loss of granule cell layer under the microabscesses, acanthosis, and elongation of the rete ridges; the superficial dermis showed a dense inflammatory infiltrate containing lymphocytes, macrophages and neutrophils, as well as dilated and tortuous blood vessels. The lesions improved for 15 days after intramuscular corticosteroid depot therapy and worsened slightly afterwards. Later, a spontaneous, progressive remission coincided with the beginning of spring and lasted until the end of summer; the skin lesions practically disappeared during this period, and the nails looked nearly normal. During the next autumn and winter only nail hyperkeratosis was present. Serum analyses showed hyperproteinaemia and hyperglobulinaemia during the outbreak phase and normal values during remission. The clinical and histopathological features of this case, as well as its evolution, are compared with the three other reported cases of psoriasiform skin lesions in nonhuman primates. To the authors' knowledge, this is the first report of a definite palmoplantar nonpustular psoriasiform dermatitis in a rhesus macaque.