Three cases of chronic extreme eosinophilia with granulomatous lesions in the liver have been studied. A larval nematode observed in sections from the liver of one patient has been identified either as Toxocara canis or Toxocara cati, common cosmopolitan ascarids of dogs and cats; available evidence favors the former. The term visceral larva migrans is proposed for this type of parasitism, known in animals but not previously described in humans. It is related to better known cutaneous larva migrans, in that both are usually caused by infective stage larvae of nematode parasites of other animals. Man being an abnormal host, either has unfavorable tissue reactions or otherwise fails to provide stimuli for usual tissue migration and development of the parasite. As a result, larvae remain active for variable periods in various tissues. Similar but less severe and less prolonged reactions occur when the larvae of normal nematode parasites of man invade the tissues of a hyperimmune individual. Visceral larva migrans is usually a relatively benign disease, characterized chiefly by sustained eosinophilia, pneumonitis and hepatomegaly, and probably is due both to direct tissue damage by migrating larvae and to allergic responses to their products. Its severity varies with the number of larvae in the tissues and the immune or allergic state of the infected individual.