Two women with typical clinical and biochemical features of pituitary-dependent Cushing’s disease each underwent hourly blood sampling for 24 h on two separate occasions for measurement of serum cortisol. The 24-h mean serum cortisol concentrations (17.7 and 15.4 μg/dl in patient 1; 19.0 and 15.8 μg/dl in patient 2) were elevated (normal level, <12.5 μg/dl), as expected. Cosinor analysis of the patients’ serum cortisol patterns revealed statistically significant circadian rhythms on all four profiles. The amplitude of the rhythm on both occasions in patient 1 (5.8 and 6.6 μg/dl) and on one of two occasions in patient 2 (7.2 and 10.5 μg/dl) fell in the range for the amplitude of the cortisol circadian rhythm in normal subjects (2.2-8.6 μg/dl). In contrast to commonly held belief, some patients with Cushing’s disease may exhibit circadian variation of serum cortisol.