The interdaily variation in plasma concentration of 11-hydroxycorticosteroids (11-OHCS) between 7.30 and 8.30 a. m. was studied in 63 patients in a general medical ward. When tested against the analytical error in every single case it was found that 44 out of 63 patients showed a significant difference between three consecutive morning values. The maximal difference varied from 2.6 to 24 μg 11-OHCS per 100 ml plasma.