In February 1987, 322 Sudanese school children were diagnosed for Schistosoma mansoni infection and treated randomly with praziquantel (either 20 mg/kg or 40 mg/kg body weight). A followup of these subjects was carried out in January 1989. This treatment resulted in a substantial reduction of egg output. Patients underwent complete abdominal ultrasonography and periportal fibrosis of the liver was graded into three degrees of severity. The proportion of patients with periportal fibrosis decreased from 36.6% in February 1987 to 21.7% in January 1989. At the time of followup, higher grades of periportal fibrosis (grades II and III) were encountered in only 4.3% and 0.3% of these patients, respectively, compared with 21.1% and 5.9%, respectively, before therapy. This was paralleled by a significant decrease in hepatomegaly from 10.9% to 7% of the patients. In contrast, the rate of splenomegaly showed a slight increase during the period of observation. The different dosage regimens of praziquantel did not result in a significantly different reversibility of periportal fibrosis or a decrease in egg excretion. The reversibility of specific liver lesions 23 months after antischistosomal therapy with praziquantel was substantial. The improvement was greater at 23 months than that obtained seven months after treatment.