Current guidelines for distinguishing Cushing's disease (CD) from ectopic ACTH secretion (EAS) are questionable, as they use pituitary MRI as first-line investigation for all patients, CRH testing is no longer available and they suggest performing inferior petrosal sinus sampling (BIPPS), an invasive and rarely available investigation, in many patients. To establish non-invasive personalized diagnostic strategies based on the probability of EAS estimated from simple baseline parameters. Retrospective study. University hospitals. 247 CD and 36 EAS patients evaluated between 2001 and 2023 in 2 French hospitals. A single-center cohort of 105 Belgian patients served for external validation. 24h-urinary free cortisol (UFC) had the highest area under ROC curve for discrimination of CD from EAS (0·96 [95% CI, 0·92-0·99] in the primary study and 0·99 [95% CI, 0·98-1·00] in the validation cohort). The addition of clinical, imaging and biochemical parameters did not improve EAS prediction over UFC alone, with only BIPPS showing a modest improvement (c-statistic index 0·99 [95% CI, 0·97-1·00]). 3 groups were defined based on baseline UFC:  10 x the upper limit of normal (group 3), and were associated with 0%, 6·1% and 66·7% prevalence of EAS, respectively. Diagnostic approaches performed in our cohort support the use of pituitary MRI alone in group one, MRI first followed by neck-to-pelvis CT-scan (npCT) when negative in group 2, and npCT first followed by pituitary MRI when negative in group 3. When not combined with the CRH test, the desmopressin test has limited diagnostic value. UFC accurately predicts EAS and can serve to define personalized and non-invasive diagnostic algorithms.