Summary A technique for the quantitative recovery of S. mansoni from human cadavers was presented. Analysis of the data from 197 autopsies indicated that nearly all worm pairs were recovered. The procedure is moderately time-consuming but not difficult, and can be performed with simple equipment and with little interference in the autopsy routine. Evidence of heavy infection was most common in cases of Symmers' clay-pipestem fibrosis of the liver. In over half of untreated cases of Symmers' fibrosis there were more than 160 worm pairs, whereas in only half of those without Symmers' fibrosis were there more than 10 worm pairs. It is postulated that a heavy infection is a prerequisite for the development of Symmers' fibrosis. Schistosomal pulmonary arteritis and cor pulmonale, when present, were associated with Symmers' fibrosis. The only other gross evidence of disease attributable to S. mansoni infection was a single case of ulcerative colitis in a massively infected child (1,608 worm pairs). Cadavers with portal or postnecrotic cirrhosis had light infections. When Symmers' fibrosis was not present, liver and spleen weights were not increased, nor were they related to the intensity of infection. The number of eggs in the feces reflected the worm burden in the absence of Symmers' fibrosis, although there was great individual variation. In cases of Symmers' fibrosis, and apparently in those of cirrhosis, fewer eggs per worm pair were found in the feces than in other cases. This, together with the mixture of lightly and heavily infected instances of Symmers' fibrosis, would make difficult ante-mortem estimation of the intensity of infection. The number of eggs per gram of feces per worm pair did not change significantly with age or intensity of infection. Routine ante-mortem fecal examinations had been frequently positive in cadavers harboring fewer than 10 worm pairs. The number of eggs in the tissues was related to the intensity of infection, but showed changes related to age and worm burden. The number of tissue eggs per worm pair increased in older persons, but decreased with increasing intensity of infection; the latter apparently reflected more rapid destruction of eggs in the tissues in heavy infection. The distribution of worms did not vary greatly with age or intensity of infection. In cases of Symmers' fibrosis, and in those of cirrhosis, an increased proportion of worms and eggs was found in the small intestine, and there was a striking decrease in the proportion of eggs in the rectosigmoid colon, the most frequent site of oviposition in subjects without Symmers' fibrosis or cirrhosis. The ratio of male to female worms recovered was slightly greater than one and showed no change with age or intensity of infection. In rare instances, male worms only were recovered; in these, few worms were found, and a small number of dead eggs was generally found in the tissues, indicating previous bisexual infection. The post-mortem data suggest that light S. mansoni infections, including those of many years' duration, are of little consequence, although obviously they do not exclude the possibility that they produce significant disease not evident at autopsy.