The majority of patients with congenital heart disease (CHD), particularly those with relevant systemic-to-pulmonary shunts, if left untreated, will experience the development of pulmonary arterial hypertension (PAH). Previous studies have shown that platelet activation occurs in patients with PAH. In response, this study aimed to investigate the platelet indices, including platelet distribution width (PDW) and mean platelet volume (MPV), in patients with PAH. The study was conducted in the pediatrics cardiology unit of Selcuk University Medical Faculty between July 2010 and January 2012. The patients’ clinical and laboratory data were obtained retrospectively from hospital recordings. The study enrolled 57 children with CHD (all with left-to-right shunting). The patients who had undergone diagnostic cardiac catheterization were analyzed according to the presence or absence of PAH. Group 1 had CHD with PAH, and group 2 had CHD without PAH. Compared with the group 2 patients, a significant decrease in PDW (p < 0.0001) was noted in the group 1 patients (with PAH). Likewise, a significant difference was found in the MPV of group 1 (p < 0.0001). Statistically, groups 1 and 2 did not differ in terms of platelet count (p = 0.3). Patients who had pulmonary hypertension secondary to CHD with left-to-right shunting exhibited a lower PDW and MPV.