Major declines of populations of caribou and reindeer (Rangifer tarandus) that permanently reside on Arctic tundra have been attributed to short-term inaccessibility of forage through restrictive snow cover. Such density-independent phenomena would produce unpredictable changes in populations of Arctic tundra caribou. In 1985, Inuit correctly predicted mass emigration from the winter range of a caribou subpopulation on Foxe Peninsula (FP), southern Baffin Island, Canada. During 1982-94, we conducted aerial surveys, satellite telemetry, and physical condition studies to examine features of the predicted range shift. Between 1984 and 1992, caribou density on upland terrain on FP dropped (P < 0.001) from 6.2 to 0.3 caribou/km2. Cows began to emigrate en masse during winter 1988-89 (P = 0.10) toward Meta Incognita Peninsula (MIP), where caribou showed greater fidelity to that wintering area during 1988-94 (P = 0.005). Density of caribou on upland terrain on MIP increased (P = 0.001) from 0.2 to 5.0 caribou/km2 between 1982 and 1992. In April 1992, body size did not differ (P - 0.47) between FP and MIP. Cows on MIP had greater (P - 0.04) fat and muscle reserves than cows on FP, while only fat reserves of MIP bulls were greater than (P - 0.03) those of FP bulls. Our results support Inuit observations of declining physical condition of FP caribou in the early 1980's, and their view that the range shift was caused by cumulative annual overgrazing of the winter range during the previous 10 to 30 yr. Fewer cows on FP were pregnant (2 of 8) than on MIP (10 of 10; P = 0.002). Calf:cow ratios were higher (P = 0.05) on MIP than on FP in 1992. Although few caribou had occupied MIP for 50 yr before 1988-89, MIP caribou were in relatively poor condition by April 1992 compared to those on overgrazed Coats Island during mild winters. Winter range shifts and population declines by Arctic tundra caribou may be predictable. Ecological indicators may enable managers to mitigate the effects of overgrazing on caribou populations through intensive harvesting at critical stages during long-term population increases.