Trends in Thick-billed Murre (Uria lomvia) populations at Prince Leopold Island and Coats Island, Nunavut—colonies at opposite ends of the species range in the eastern Arctic—were compared over the period 1985–2000. Population trends were monitored by daily counts of fixed study plots on six or more days each year. At Coats Island, annual mean counts were well correlated with numbers of breeding pairs located on separate breeding study areas, suggesting that the monitoring counts provided a useful index of the breeding population. Overall, counts at both colonies increased over the period of observations (by 2.1% annually at Coats Island and 1.5% at Prince Leopold Island), but a period of significant decline occurred during 1989–1991 and numbers remained stable after 1998. Fluctuations at the two colonies were well-synchronized. Changes in numbers from year-to-year were positively correlated with the mean mass of breeders during the first half of incubation. Hence, birds appeared to be in poorer condition in years when the population decreased. The similarity in fluctuations at colonies as far apart as Coats and Prince Leopold islands suggests that population changes may be determined by events on the common wintering grounds. The correlation between changes in counts and body mass at Coats Island suggests that the common factor may be one that affects the availability of food during the nonbreeding period.