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Midnight salivary cortisol (MSC) is now recognized as a reliable index for Cushing’s syndrome diagnosis but has to be validated for the follow-up of treated patients.Our objective was to evaluate MSC for assessing the outcome of transsphenoidal surgery (TSS) in patients with Cushing’s disease (CD).We conducted a retrospective cohort study in a single center.Sixty-eight patients treated by TSS between 1996 and 2006 and followed for at least 6 months with postoperative MSC were included. Mean follow-up (± sd) was 45 ± 31 months. Morning plasma cortisol was determined 5 d after TSS, and MSC and urinary cortisol (UC) were determined 6-12 months after surgery. The remission group included hypocortisolic (morning plasma cortisol < 50 ng/ml and/or insufficient response to cosyntropin) and eucortisolic (midnight plasma cortisol < 75 ng/ml and normal UC) patients. Patients in the treatment failure group had high midnight plasma cortisol and UC concentrations.Fifty patients (74%) were in remission. Mean MSC was 0.7 ± 0.4 ng/ml (range, 0.4-2.1 ng/ml) and 6.5 ± 6.5 ng/ml (range, 2.1-27.2 ng/ml) for the remission and treatment failure groups, respectively (P = 0.001). A cutoff of 2 ng/ml for MSC gave a sensitivity of 100% and a specificity of 98% for treatment failure diagnosis, whereas UC less than 90 μg/d had a sensitivity of 71% and specificity of 98%. Postsurgical morning plasma cortisol less than or equal to 18 ng/ml had a sensitivity of 93% and specificity of 74%.MSC is a simple, robust marker of remission after TSS for CD.