OBJECTIVE: Since Cushing's disease due to large pituitary tumors is rare, we evaluated biochemical characteristics at entry and the results of first surgical approach and of adjuvant therapeutic strategies during a long-term follow-up period. DESIGN: We studied 26 patients (nine male, 17 female; 42.5+/-12.7 years, mean+/-s.e.) with ACTH-secreting pituitary macroadenoma (tumor diameter: 11-40 mm). METHODS: At entry, plasma ACTH, serum cortisol and 24-h urinary free cortisol (UFC) levels were measured in all patients, a high-dose dexamethasone (dexa) suppression test was evaluated in 22 cases and a corticotrophin releasing hormone (CRH) test in 20 cases. Patients were re-evaluated after operation and, when not cured, they underwent second surgery, radiotherapy and/or ketoconazole treatment. The follow-up period was 78+/-10 months. RESULTS: Before surgery, dexa decreased ACTH (>50% of baseline) in only 14/22 patients. The CRH-stimulated ACTH/cortisol response was normal in six patients, impaired in six patients and exaggerated in eight patients. After operation eight patients were cured, nine had normalized cortisol levels and nine were not cured. Pre-surgery, mean ACTH values were significantly higher in the not cured patients than in those normalized (P<0.05) and cured (P<0.01); the ACTH response to CRH was impaired in only six patients of the not cured group. The tumour diameter was significantly less in cured patients (P<0.02) and in normalized patients (P<0.05) than in the not cured ones. Magnetic resonance imaging (MRI) showed invasion of the cavernous sinus in 2/9 normalized, and in 6/9 not cured patients. After surgery, ACTH, cortisol and UFC were significantly lower than at entry in cured and in normalized patients, but not in not cured patients. In the cured group, the disease recurred in one patient who was unsuccessfully treated with ketoconazole. In the normalized group, a relapse occurred in eight patients: radiotherapy and ketoconazole induced cortisol normalization in one case, hypoadrenalism in one case and were ineffective in another one, while five patients were lost at follow-up. In the not cured group, eight patients underwent second surgery, radiotherapy and/or ketoconazole, while one patient was lost at follow-up. These therapies induced cortisol normalization in two patients and hypoadrenalism in one. CONCLUSIONS: (i) A sub-set of patients with ACTH-secreting pituitary macroadenoma showed low sensitivity to high doses of dexamethasone and to CRH, (ii) pituitary surgery cured Cushing's disease in a minority of patients, (iii) high baseline ACTH levels, impaired ACTH response to CRH, increased tumor size or invasion of the cavernous sinus were unfavourable prognostic factors for surgical therapy, and (iv) second surgery, radiotherapy and/or ketaconazole cured or normalized hypercortisolism in half of the patients with recurrence or not cured.