The adverse reactions mentioned in 267 reports on levamisole are analyzed. Significant adverse reactions are agranulocytosis, skin rash, and febrile illness. They occur in a small subpopulation of patients only, predominantly those with rheumatoid arthritis. Other side effects rarely necessitate discontinuation of treatment. Levamisole did not appear to cause renal or liver toxicity. By means of a questionnaire, cases of blood dyscrasia were analyzed in detail. Agranulocytosis (less than 20% neutrophils) occurs most frequently in patients with rheumatic diseases, in women, and in HLA B27 genotypes. It is spontaneously reversible when treatment is discontinued. The bone marrow is not damaged. Thrombocytopenia has a course similar to that of agranulocytosis but it is very rare. Leukopenia (less than 3000 cells/mm3 but greater than 20% neutrophils) seems to differ from agranulocytosis in a number of ways and is not an indication to discontinue treatment. It is concluded that, if used under the close supervision of a physician, for those clinical situations in which levamisole has proven efficacy, the potential benefit to the patient outweighs the hazards and justifies its use.