Larva migrans in man is a disease characterized by inflammatory reaction around or in the wake of migrating larvae, most commonly larvae of nematode parasites of other animals. In the skin the condition is called cutaneous larva migrans; in the deeper tissues, visceral larva migrans. Cutaneous larva migrans is caused by the larvae of dog and cat hookworms, Ancylostoma braziliense and A. caninum; less often by Uncinaria stenocephala of dogs and Bunostomum phlebotomum of cattle. Ancylostoma ceylanicum, formerly confused with A. braziliense, also may be involved. Transient but otherwise typical cutaneous larva migrans lesions may be produced by the human hookworms, Ancylostoma duodenale and Necator americanus, and by Strongyloides stercoralis. Visceral larva migrans is best known in the form produced by the larvae of Toxocara canis, these having been identified in autopsy specimens of lungs, liver, brain or other organs in three instances, in biopsy specimens of the liver on numerous occasions, and in several enucleated eyes. The larvae migrate extensively before encapsulation and may persist without morphological change for more than a year. Toxocara cati and other ascarids probably infect man in much the same manner. The non-human hookworms that produce cutaneous larva migrans subsequently migrate to the viscera. The larvae of A. caninum persist in the tissues as long as those of Toxocara canis. Although Capillaria hepatica reaches sexual maturity in the tissues of the liver, the infection is otherwise similar to visceral larva migrans and may, therefore, be included among the occult nonpatent nematodiases. Gnathostoma spinigerum which reaches its normal location in the wall of the stomach of dogs and cats by direct migration through the liver and other tissues rarely, if ever, reaches maturity in man. Instead it migrates to various other sites including both skin and viscera. Less frequently, similar infections of other spiruroids such as species of Gongylonema, Habronema, Physaloptera, Chilospirura and Thelazia have been observed. Dirofilaria spp. and perhaps some other filarial worms of wild or domesticated animals occasionally undergo considerable development in man, producing transient migrating areas of urticaria, inflammation, or abscesses. For some of the larva migrans producing larvae, man is merely an accidental but more or less normal intermediate or paratenic host. Examples are Toxocara canis and Toxocara cati. Possibly also to be included in this category are Ancylostoma caninum, A. braziliense and some of the spiruroids. For the larva migrans producing filarial species man is a final host, clearly abnormal. To the dog and cat hookworms, A. caninum, A. braziliense, and A. ceylanicum, man's relationship is apparently that of an abnormal final host, but, for A. caninum at least, experimental evidence strongly suggests a facultative intermediate host relationship. For Capillaria hepatica, as for Trichinella spiralis and other species lacking host specificity, man may be regarded as an unnatural but normal host. For his natural parasites such as Necator americanus, and others which occasionally produce larva migrans-like reactions in the skin or other organs, he may be regraded as a natural, and normal, but hypersensitized host; the term “natural” being used for relationships or hosts usually involved in maintaining the parasite as a species in nature, while the term “normal” denotes relationships and hosts that may serve quite satisfactorily in this regard, but are not depended upon in nature to maintain the parasite as a species.