Aplastic anemia (AA) is a classic bone marrow failure syndrome simply defined as peripheral blood pancytopenia and a hypocelular bone marrow, yet the diagnosis must be made by excluding other causes of bone marrow failure. The incidence rate of AA reported by the International Aplastic Anemia and Agranulocytosis Study (IAAAS) in the 1980s was 2 cases per 1 million people. This disease is known to be caused by exposure to radiation, chemotherapy and some viral agents, yet most of the cases are idiopathic. Epstein Barr virus and non-A, non-B or non-C Hepatitis virus have classically been related to the development of some AA cases. Recently there have been some reports of AA following Parvovirus B19 (PvB19) infection. This virus, the only parvoviridae virus capable of infecting humans, attacks erythrocyte precursors attaching to the P antigen in their surface and requiring Beta1 integrin for viral entry. Although PvB19 seems to infect only erytroid precursors, it is widely recognized that the infection with this virus can cause not only anemia, but neutropenia and thrombocytopenia as well, producing aplastic crisis of varying intensity. A correlation has recently been found between PvB19 DNA in peripheral blood and AA in children. We pretend to corroborate this observation and include adult patients in order to improve our understanding of the relationship between PvB19 and AA. So far we have taken peripheral blood samples from 9 AA patients and 9 controls paired by age, sex and community; we plan to include 100 AA patients and their controls from several hospitals around Mexico. DNA was extracted using the PUREGENE DNA extraction kit (Gentra, Minneapolis MN). Nested PCR was performed using the sense primer (P1) 5-AATACACTGTGGTTTTATGGGCCG-3, antisense (P2) 5-CCATTGCTGGTTATAACCACAGGT-3 for the first round and the sense primer (P3) 5-AATGAAAACTTTCCATTTAATGATGTAG-3 and antisense primer (P4) 5-CTAAAATGGCTTTTGCAGCTTCTAC-3for the second round. A DNA sample from a patient with active infectious mononucleosis with positive IgG and IgM against PvB19 in serum was used as positive control. Two samples from the AA group (22%) and 1 from the control group (11%) have turned positive for PvB19 DNA. The reported incidence for the presence of this virusDNA in the peripheral blood of the population is 3%. We expect that, as the number of patients grows, the percentage of positive samples in the control group will decrease, while the percentage of positive samples in the AA group will rise or be sustained. Our partial results point towards a possible relationship between AA and the presence of PvB19 DNA in the peripheral blood cells. It is possible that this virus is one of many factors capable of precipitating the development of AA by limiting the bone marrows capacity to produce blood cells. We are in the process of gathering more samples to prove if a relationship really exists and, if so, future studies will likely shed light upon the mechanism by which PvB19 contributes to the development of AA and other marrow failure syndromes.