The objective of this investigation was to describe the epidemiology of anotia and microtia with respect to various factors. The cases studied were all infants and fetuses with anotia or microtia identified by a population-based birth defects registry in Hawaii. The anotia and microtia rates were determined for selected factors and comparisons made among the subgroups by calculating the rate ratio (RR) and 95% confidence interval (CI). A total of 120 cases were identified, for a rate of 3.79 per 10,000 live births. The anotia and microtia rate increased during 1986-2002, although the trend was not significant (P = 0.715). Of 49 specific structural birth defects examined, four were found to be significantly more common in the presence of anotia and microtia. When compared with Caucasians, the anotia and microtia rates were higher among Far East Asians (RR 1.79, 95% CI 0.89-3.68), Pacific Islanders (RR 2.26, 95% CI 1.24-4.32), and Filipinos (RR 2.34, 95% CI 1.23-4.64). The defects were less common among females (RR 0.64, 95% CI 0.43-0.93) and more common with multiple birth (RR 3.72, 95% CI 1.66-7.33), birth weight < 2500 g (RR 3.35, 95% CI 2.04-5.30), and gestational age <38 weeks (RR 2.27, 95% CI 1.49-3.40). In conclusion, the rate for anotia and microtia increased in Hawaii during the study period. The rates for only a few structural birth defects were substantially greater than expected in association with anotia and microtia. Anotia and microtia rates varied significantly according to maternal race/ethnicity, infant sex, plurality, birth weight, and gestational age.