We report a case of a woman, born in 1943, with definite ankylosing spondylitis (AS) fulfilling the revised New York criteria for AS, although human leukocyte antigen B 27 negative. After 20 years of evolution, she developed a febrile skin disease corresponding to Sweet's syndrome, confirmed by skin biopsy. No other underlying condition was found. The relationship between AS and Sweet's syndrome is discussed; only one other case is reported in the literature. Skin involvement in AS is dominated by psoriasis-type lesions. The occurrence of Sweet's syndrome is seldom reported in this condition.