Visceral larva migrans is a disease in which the larvae of animal parasites invade human tissues but do not complete their life cycles. The most frequent cause of this illness in children is the dog roundworm, Toxocara canis. A review of the literature, as well as our clinical experience, indicates that there are two forms of clinical expression: one, visceral, and the other, ocular. In general the clinical and laboratory abnormalities (hepatomegaly, recurrent pneumonia, eosinophilia, and hyperglobulinemia) usually associated with visceral disease are absent in children with ocular abnormalities. Conversely, there is a general lack of eye complications in patients with systemic disease. Reasons for these variations in clinical expression are unknown, but immune responses of the host and the antigenic composition of the parasite may be contributing factors.