In this study, we adopted the AA investigational assessment guidelines proposed by Olsen et al. The patients with AA were divided into four groups by age of onset of AA [childhood (age: 0–15), adolescence (age: 16–19), early-adulthood (age: 20–39), and late-adulthood (age: 40≦)], and then analyzed. The study population (total 205 cases) included 136 females (66.3%) and 69 males (33.7%) with a male/female ratio of 1: 1.9. The age of onset ranged from 1 to 73 years. The mean age of onset was 29.7±17.1 years in males and 34.5±17.6 years in females (p=0.0704). One-hundred twenty (58.5%) out of 205 patients had limited alopecia (S1–2, less than 50% scalp hair loss); 61 patients (29.8%) had moderate alopecia (S3–4, 50–99% scalp hair loss); 5 patients (2.4%) had alopecia totalis; and 19 patients (9.3%) had alopecia universalis. The distribution of cases by sex and age of onset showed two peaks between 20 and 24 years and between 50 and 54 years in females, while no significant peaks were seen in males. The month of onset was identified in 147 out of 205 patients, and the distribution of cases by month of onset showed that the incidence of onset increasd in April and decreasd in November. One-hundred and five (71.4%) out of 147 patients were included in the 7 months from April to October. The childhood group consisted of 18 females and 19 males with a female/male ratio of 0.95:1; the adolescence group consisted of 9 females and 5 males with female/male ratio of 1.8:1; the early-adulthood group consisted of 57 females and 24 males with female/male ratio of 2.4:1; and the late-adulthood group consisted of 52 females and 21 males with female/male ratio of 2.5:1, suggesting that the rate of female/male ratio may be high in the late onset cases (p=0.0876). The rate of extensive alopecia (S3–4≦) was 56.8% in the childhood group, 64.3% in the adolescence group, 43.2% in the early-adulthood group, and 27.4% in the late-adulthood group, suggesting that patients with early onset showed more severe types (p=0.0057). Positive family histories of AA were seen in 46 (22.4%) out of 205 patients, and the positive rate of a family history of AA was significantly higher in females than that in males (male 13.0%, female 27.2%, p=0.0216). The positive rate of a family history of AA was 27.0% in the childhood group, 21.4% in the adolescence group, 24.7% in the early-adulthood group, and 17.8% in the late-adulthood group (p=0.6631). The rate of positive past and/or present histories of atopic dermatitis (AD) was 40.5% in the childhood group, 35.7% in the adolescence group, 23.5% in the early-adulthood group, but there were no patients with positive histories of AD in the late-adulthood group, suggesting a high incidence of AD in the patients with early onset (p<0.0001). The rate of positive anti-thymoglobulin antibody was 13.5% in the childhood group, 21.4% in the adolescence group, 17.3% in the early-adulthood group (p=0.5019), and 24.7% in the late-adulthood group. In this study, it was suggested that the clinical features of AA are distinguished by sex and the age of onset, and it may reflect heterogeneity of the mechanisms in the development of AA.