Context Mild autonomous cortisol secretion (ACS) is associated with an increased risk of vertebral fractures (VFx). However, the influence of this condition on bone turnover or its association with mild ACS is still controversial.This study aimed to evaluate the impact of mild ACS on bone quality among patients living with the disease.A retrospective study was conducted using data from 55 mild ACS and 12 nonfunctioning adrenal tumor (NFT) patients who visited Chiba University Hospital, Japan, from 2006 to 2018.We analyzed clinical features and bone-related factors, including bone mineral density (BMD) and VFx, performed blood tests to assess bone metabolism markers in patients with mild ACS and NFT, and assessed the associations between bone-related markers and endocrinological parameters in patients with mild ACS.No significant differences between mild ACS and NFT patients were observed with respect to the presence or absence of VFx and BMD. Urinary free cortisol (UFC) was higher in mild ACS patients with VFx than those without (P = 0.037). The T-score and young adult mean (YAM) of the BMD of the femoral neck in mild ACS patients with a body mass index < 25 were positively correlated with dehydroepiandrosterone-sulfate levels (ρ: 0.42, P = 0.017; ρ: 0.40, P = 0.024, respectively). Pearson's correlation analysis showed that bone-specific alkaline phosphatase was negatively correlated with UFC in the patients with mild ACS (ρ: -0.37, P = 0.026).These results suggest that urinary free cortisol may be useful for predicting bone formation in mild ACS patients.