In recent years, introduction of new and more effective agents has improved the overall therapy for parasitic infections. This field, however, is still plagued by numerous problems, including the development of resistance to antimicrobial agents (especially with malaria), unavailability of agents in the United States or lack of approval by the Food and Drug Administration, and major toxicities or lack of experience in pregnant women and children, which limits use in these groups of patients. Widespread resistance of Plasmodium falciparum to chloroquine and other agents has complicated the treatment and prophylaxis of this type of malaria. A combination of quinine and Fansidar is usually effective oral therapy for falciparum malaria; quinidine may be administered if intravenous therapy is needed. Mefloquine, which is currently recommended for prophylaxis against chloroquine-resistant P. falciparum, is also effective for single-dose oral treatment, although this regimen has not yet been approved by the Food and Drug Administration. Metronidazole has been widely used for treatment of gastroenteritis due to Entamoeba histolytica and Giardia lamblia (not approved by the Food and Drug Administration for the latter) and is considered safe and effective. A new macrolide, azithromycin, has been reported to be effective for cryptosporidiosis in experimental animals; currently, no effective therapy is available for human infections. Combinations of sulfonamides with other antifolates, trimethoprim or pyrimethamine, are recommended therapy for Pneumocystis carinii pneumonia or toxoplasmosis, respectively. Therapies for the various types of leishmaniasis and trypanosomiasis are complex, often toxic, and often of limited efficacy. The benzimidazoles are effective for roundworm infections, although thiabendazole has severe toxic effects. The recent introduction of ivermectin has revolutionized the treatment and control of onchocerciasis. Another relatively new agent, praziquantel, is a true broad-spectrum anthelmintic agent that is effective against most trematodes, many adult cestodes, and larval cestodes as well (especially cysticerci of Taenia solium).