Summary and Conclusions 1. Falciparum malaria in the drug addict is an endemic disease in the New York metropolitan area. 2. Infection is direct from man to man and results from the practice of sharing common hypodermic syringes for intravenous heroin administration. 3. Clinically and pathologically the disease manifests all the characteristics of falciparum malaria as it occurs in the tropics including various cerebral, gastrointestinal, and hemoglobinuric syndromes. 4. The outstanding diagnostic feature is the history or evidence of drug addiction. On this evidence obscure acute syndromes in drug addicts warrant specific therapy until a diagnosis other than malaria is established. Repeated efforts to demonstrate malarial parasites in the peripheral blood should be made. 5. Early diagnosis and intensive therapy must coexist if a high mortality rate is to be avoided. Treatment should include an effective gametocidal drug to safeguard the health of non-immunes where malaria, suitable anophelines, or addicts are abundant.