During routine antimalarial drug efficacy trials, we observed, for the first time, severe thrombocytopenia developing in Aotus monkeys infected with Plasmodium vivax. Data obtained from 26 Aotus infected with the AMRU-1 strain showed that 77% developed severe thrombocytopenia, whereas only 15% had severe anemia, with hemorrhagic diathesis ensuing in 31%. In general, thrombocytopenic monkeys either failed primary treatment with experimental antimalarial drugs or were found to have higher-density parasitemias, longer patency duration, and lower hematocrits. In these monkeys, severe thrombocytopenia inversely correlated to parasitemia (R = -1.0), and animals that received a blood transfusion had significantly higher platelet counts (P < 0.05) by day 38 after inoculation. In conclusion, the AMRU-1 strain of P. vivax, was considered to be highly pathogenic to Aotus monkeys, and thrombocytopenia rather than anemia should be regarded an early indicator of drug treatment failure with this strain.