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The aim of this report is to describe five patients with lamivudine-induced pure red cell aplasia, an association not previously described. We describe patients with unresponsive anemia in whom a complete study including blood cell counts, reticulocyte counts, hemolysis tests, and bone marrow aspiration was performed. Pure red cell aplasia was considered when anemia was associated with normal leukocyte and platelet counts with a corrected reticulocyte count below 1% and less than 5% bone marrow erythroid progenitors in the absence of positive hemolysis tests. Complete remission was considered when bone marrow erythroid progenitors were at least 16%. Five male patients had pure red cell aplasia with a median age of 32 years (range 29 to 37 years). Before lamivudine, they had hemoglobin >11.8 g/dl without transfusion requirements. After receiving the drug, hemoglobin dropped to 5.2 g/dl (4.3 to 6.1 g/dl) with high transfusion requirements and mean bone marrow erythroid progenitors of 1.84% (0 to 4%). Withdrawal of lamivudine was attempted to confirm the diagnosis. Seven weeks after stopping lamivudine, hemoglobin rose up to 12.8 g/dl (11 .3 to 13.8 g/dl) and bone marrow erythroid progenitors increased up to 25.6% (21 to 40%) without transfusion requirements. Lamivudine-induced pure red cell aplasia may be a cause of anemia unresponsive to conventional treatment in AIDS. Since lamivudine use in Mexico has been relatively short, we expect more cases to appear in the future.