Although adult plumage in Cooper's Hawks (Accipiter cooperii) exhibits little or no sexual dichromatism, iris color reportedly changes from yellow or light orange in younger birds to shades of orange or red in older birds, especially in males. However, there is little quantitative data on this phenomenon. It has been suggested that male eye color may serve as a signal of age and hence reproductive fitness and thus offer a basis for nonrandom mating in Cooper's Hawks. In this study we examine the relationships between eye color and age, sex, and reproductive output for Cooper's Hawks in two breeding populations in British Columbia and North Dakota, 1999–2002, and compare these results to those previously published for a breeding population in Wisconsin, 1980–1995. Cooper's Hawks in British Columbia and North Dakota appear to acquire darker orange or red irides more frequently and more quickly than their counterparts at known and relative ages in Wisconsin. Females in all study sites are slower and less likely than males to acquire the darkest eye colors. Eye color is not a reliable predictor of age in individual male and female Cooper's Hawks, for researchers and perhaps for the birds themselves, because individual hawks of a given eye color displayed variation in known and relative ages in British Columbia and Wisconsin. There was no significant relationship between the eye color of males and their brood sizes in any of these three populations, and therefore no discernable support for the premise that male eye color per se signals male fitness, or functions as a sexual trait for assortative mating in this species.