Congenital syphilis is often a presumptive diagnosis (based on serologies), because confirmation requires identification of Treponema pallidum in fetal/neonatal tissues or in the placenta. Placental histological features associated with congenital syphilis include the triad of enlarged hypercellular villi, proliferative fetal vascular changes, and acute or chronic villitis. The authors blindly evaluated 49 formalin-fixed, paraffin-embedded placentas (38 with positive maternal syphilis serologies; 11 with negative serologies) and compared results of histology, Steiner stain, and polymerase chain reaction (PCR) for T pallidum DNA. Histology was categorized as positive (triad present), suspicious (two thirds of triad present), or negative. Treponemal DNA was detected by amplifying a 189 base pair region of the 47 kd treponemal membrane antigen with 44 cycles of PCR; products were detected by Southern blot. Placentas from the 11 seronegative mothers were all negative by histology, Steiner stain, and PCR. Among the 38 placentas from serologically positive mothers, 4 had positive histology (2 of 4 positive Steiner, 4 of 4 positive PCR); 6 had suggestive histology (0 of 6 positive Steiner; 1 of 6 positive PCR); and, 28 had negative histology (0 of 28 positive Steiner; 1 of 28 positive PCR). PCR identification of treponemal DNA was significantly associated with the triad (P = .0003), proliferative fetal vascular changes (P = .0003), acute villitis (P = .003), chronic villitis (P = .004), and spirochetes on Steiner stain (P = .01). These results (1) confirm a strong association between placental histopathologic features and congenital syphilis; (2) indicate that when such features are present, PCR of placental tissue may confirm the diagnosis of congenital syphilis; and (3) suggest that even when such features are absent, PCR of placental tissue may identify additional cases of histologically unsuspected congenital syphilis.