It has been reported that both chorangiosis and chorangioma are increased in placentas at high altitudes. In this study, 1.14% of 2448 Japanese placentas obtained at 30-300 m had chorangiosis or chorangioma, compared with 3.24% of the Nepalese placentas (1300-3000 m) and 9.09% of the Tibetan samples (3800-4200 m). The incidence of both pathologies was significantly higher in the Himalayan groups than those of the Japanese group (P < 0.05). Obstetric complications of the 28 Japanese placentas with chorangiosis and chorangioma included Caesarean section 16 (57.1%), abruptio placentae 2 (7.14%), intrauterine growth retardation 3 (10.7%), intrauterine fetal death 4 (14.2%) and placenta praevia 4 (14.2%). Four infants (14.2%) had Apgar scores ranging from 0 to 3. Among 48 Himalayan placentas with chorangiosis and chorangioma, incidence of marked subchorionic fibrin was significantly higher (25%) than in the Japanese group (3.57%) (P < 0.05). The incidence of abnormal insertion of the cord was significantly higher (14.2%) in the Japanese group than in the Himalayan group (5%) (P < 0.05). There was no significant difference in the frequencies of intervillous thrombosis and infarction between the two groups. These findings suggest that the incidence of pathological change such as chorangiosis and chorangioma may be increased in placentas at Himalayan altitudes.