The secondary cutaneous manifestations of the initial phase of schistosomiasis mansoni were studied in 34 patients who had been infected simultaneously in the same location. Sixteen of the patients developed angioedema and/or urticaria, generally of short duration and of mild intensity and usually about 30 days post-infection. There was no apparent association between the occurrence of these skin manifestations and the patients' worm burdens, blood eosinophilias or areas of immediate reaction to an intradermal inoculation with worm antigen. Other factors, particularly host immunological reactivity, are thought to be important elements in the genesis of the manifestations.