We assessed the value of postoperative plasma cortisol concentrations to predict cure and recurrence of Cushing's disease after transsphenoidal surgery (TS). Seventy-eight of 80 consecutive patients treated by TS for Cushing's disease were evaluated. TS cured 72% (n = 56) of the patients. Two weeks after surgery, patients with plasma cortisol levels below 138 nmol/liter (n = 50; three macroadenomas) and eight (27%) of 30 patients (nine macroadenomas) with cortisol greater than 138 nmol/liter were cured. Six (five with a macroadenoma) of these eight patients had cortisol values less than 50 nmol/liter 3 months after surgery. Therefore, the optimal cut-off value of cortisol predicting remission was 138 nmol/liter, measured 3 months after surgery (positive and negative predictive values 87 and 90%, respectively). Five patients (9%) had recurrent Cushing's disease during a median follow-up of 7 yr. Recurrence occurred in four of 24 (17%) patients with a follow-up of more than 10 yr. Therefore, cortisol levels above 138 nmol/liter, obtained 2 wk after TS, should be repeated, because they do not predict persistent Cushing's disease in 27% of those patients. Postoperative cortisol levels do not positively predict recurrence of disease during long-term follow-up of initially cured patients.