The two major cutaneous expressions of infective states are infections of the skin by viable organisms and immunological responses to nonviable microbial antigens or, in the case of molecular mimickry, their human analogues. These immunological responses are designated as cutaneous id reactions, and manifest a histomorphology similar to that seen at the primary infective site. This study presents the clinical and histological findings in 16 patients who developed skin eruptions associated with extracutaneous or systemic infections. There was a striking female predominance; patients ranged in age from 10 to 78 years. The majority of cases manifested skin lesions which clinically resembled Sweet's syndrome, erythema multiforme and/or erythema nodosum. Fever, arthralgia, oligoarthritis, mucosal ulcers of the mouth and/or genital tract and uveitis were additional features in some cases. Isolated clinical presentations included a petechial rash in a stocking and glove distribution, papular dermatitis, a morbilliform eruption and annular erythema. Among the medical and family histories were atopy and stigmata associated with connective tissue disease (CTD). Two patients were ingesting drugs with known immune dysregulating properties. Skin biopsies showed focal lymphocytic interface dermatitis, a diffuse interstitial histiocytic infiltrate, and a mononuclear cell predominant vascular reaction which in some cases represented vasculitis by virtue of manifesting concomitant luminal or mural fibrin deposition. Eosinophils, eczematous alterations, and papillary dermal edema were identified in a minority of cases. All patients had evidence of a prior or concurrent infection, based on either positive IgM serology for specific microbes or cultures. Among the implicated pathogens were cytomegalovirus, parvovirus B19, streptococcus, mycoplasma, klebsiella, and Borrelia burgdorferi. All of these organisms are among those associated with reactive arthritis, a phenomenon that was seen in some cases. The histology suggested florid cell mediated immunity (CMI), which the authors attributed to the superantigen properties held by the aforesaid pathogens. Skin lesions and constitutional symptoms resolved quickly with antimicrobial therapy in 7 of 9 cases causally linked to bacteria. Spontaneous resolution occurred in 5 of 6 virally mediated eruptions. The other 4 patients were given topical steroids or prednisone; these included 1 patient with Borrelia burgdorferi infection and 1 patient with radiographic evidence of pneumonia who was never cultured, 1 patient with parvovirus B19 infection, and 1 patient with pneumococcal pneumonia and concomitant sarcoidosis. It is the authors' belief that the eruptions seen in these patients may in part reflect a genetic or iatrogenic predisposition to respond excessively to certain infectious triggers.