Unilateral absence of a pulmonary artery (UAPA) is a rare anomaly. Although UAPA has been reported previously, its age-related pathogenesis and symptoms remain unclear. This retrospective cohort study included cases of UAPA reported in Japan at medical meetings or in the literature from 1990 through 2009. Patients with other congenital cardiac defects were excluded from the study. Clinical status was assessed according to age, and the clinical course of patients with isolated UAPA was compared with that of patients who had UAPA with a patent ductus arteriosus (PDA). Of the 92 patients with UAPA identified, 78 had isolated UAPA (14 with PDA). Hemoptysis and collateral arteries were observed in 0 and 13% of patients with isolated UAPA who were younger than 1 year, as compared with 24 and 50% of those 20 years of age or older, respectively. Pulmonary hypertension was present in 5% of the patients aged 1 to 19 years. Among patients 20 years or older, however, 32% had pulmonary hypertension, and 8% died. Compared with isolated UAPA, UAPA with PDA was associated with an earlier diagnosis (median age, 20 vs. 0 years; p = 0.002), a higher prevalence of pulmonary hypertension (22% vs. 86%; p < 0.0001), and a higher mortality rate (4% vs. 21%; p = 0.046). Collateral artery formation and pulmonary hypertension progress with age in patients with UAPA. Early diagnosis and revascularization may prevent the age-related progression of UAPA.