Malignant syphilis is an infrequent manifestation of syphilis in HIV infected patients. Only 21 cases have been published in medical literature. Most frequent in men, 62%, the mean age of presentation is 34 years. The most frequent symptoms are: 100% ulcerative cutaneous lesions, 47% fever and 33% ocular affectation. About 80% of the patients have a CD4 count >200. Lues serological test was 100% positive and 81% responded to penicillin. We report a 18 year old woman diagnosed of HIV infection, admitted to our service because of fever, painful oral sores, over-elevated eritematous lesions and cratered ulcers all over the body, with the initial suspicion of chickenpox. Lues serology turned out to be positive, as well as the Warthin Starry stain. After penicillin treatment was initiated, skin lesions improved, although residual lesions currently persist. Malignant syphilis should be considered in infected HIV patients with fever and ulcerated skin injuries as a possible diagnosis. Eventhough serological tests allow diagnosis confirmation, Warthin Starry stain can be useful when serology is negative. The first choice of treatment is penicillin and in case of penicillin allergy, a third generation cefalosporine should be used.