Introduction Childhood cancer survivors (CSs) might face an increased lifelong risk of lung function impairment. The Lung Clearance Index (LCI) has been described as being more sensitive than spirometry in the early stages of some lung diseases. The aim of this study was to evaluate this index in a cohort of patients with a history of childhood cancer for the first time. Materials and Methods We evaluated 57 off-treatment CSs aged 0–18 years old and 50 healthy controls (HCs). We used the multiple breath washout (MBW) method to study LCI and spirometry. Results CSs did not show any differences from the controls in ventilation homogeneity (LCI 6.78 ± 1.35 vs. 6.32 ± 0.44, P: ns) or lung function (FEV1 99.9 ± 11.3% vs. 103.0 ± 5.9% of predicted, P: ns; FVC 98.2 ± 10.3% vs. 101.1 ± 3.3% of predicted). LCI significantly correlated with the number of years since the last chemotherapy (r = 0.35, P < 0.05). Conclusions Our study describes the trend of LCI in a cohort of CSs and compares it with the results obtained from healthy controls. The results show that patients maintain both good values of respiratory function and good homogeneity of ventilation during childhood. Moreover, the LCI identifies the tendency toward pulmonary fibrosis, which is typical of adult CSs, at an earlier time than spirometry.