The human helper virus-dependent parvovirus, adeno-associated virus (AAV) has never been associated with disease in humans [Berns et al.(1987): Advances in Virus Research 32: 243–306; Siegl et al.(1985): Intervirology 23: 61–73]. However, in pregnant mice, infection with AAV induces early abortion [Botquin et al.(1993): Journal of Cancer Research and Clinical Oncology 119: 24]. We investigated whether this common human virus may be found in human genital tissue or in curettage material from spontaneous abortion. Using the polymerase chain reaction (PCR) AAV type 2 DNA was amplified in histological sections of 19 of 30 biopsies of the uterine mucosa. In addition, AAV-2 DNA was detected in abortion material during the first trimester of pregnancy (12/30 cases were positive) but not in material of abortion from the second or third trimester (9 cases). Whereas in tissues from the uterus AAV DNA was found only by PCR, large amounts of viral DNA were detectable by Southern blot analysis in abortion material. In situ hybridization revealed DNA of AAV to be present in the villous moiety (trophoblast) of the placenta but not in the embryo or decidua. In the same cells, AAV proteins (including the replication-associated rep proteins) were detected by immunofluorescence analysis. These results suggest (1) that AAV infects the uterine mucosa (possibly persistently) and (2) that it can replicate in trophoblast cells. This might disturb placenta development and may play a role in early miscarriage. To assess an immunological correlate of AAV infection of cervix uteri lesions and miscarriage, an enzyme-linked immunosorbent assay (ELISA) was developed for detecting serum IgG-and IgM-class antibodies to AAV-2. Compared to a control population, a significantly higher prevalence of IgM type antibodies to AAV-2 was found in women with spontaneous abortion and cervical intraepithelial neoplasia. Since about 80% of the population are seropositive for AAV-2 the high prevalence of IgM-class antibodies might be associated with reactivation of latent AAV or to reinfections due to a decline of IgG-mediated immunity. In view of the presence of AAV-2 DNA and proteins in placenta tissue, serological tests might be useful to assess further the hypothesis of a possible role of AAV infection in spontaneous abortion.