The complications of ascariasis encountered in 437 cases accessioned at the Armed Forces Institute of Pathology are described. Thirty-eight patients had serious or fatal illnesses caused by Ascaris lumbricoides. The adults, the eggs, or the larvae may cause the complications, which are quite distinct for each, in both clinical and histologic appearances. Masses of adult worms may obstruct or perforate the intestine, or individual adults may migrate into biliary ducts, causing obstruction and carrying with them bacteria that cause suppurative cholangitis and liver abscesses. Ascaris eggs in tissue incite granulomatous inflammation. Larvae migrating through the lungs of the host cause pneumonia by damaging tissue physically and stimulating an inflammatory infiltrate with abundant eosinophilic leukocytes. The criteria necessary for specific identification of nematode larvae in tissue sections are discussed.