Plasma levels of 17-hydroxycorticosteroids (17-OH-CS) during the night and day were studied in 22 cases of Cushing's syndrome. The normal diurnal variation was lacking in most of these patients; instead, the night levels were higher than normal. The mean concentration of plasma 17-OH-CS often differed markedly between different days, but the absence of diurnal variation persisted even in the presence of a lower level of plasma steroids. In 3 subjects, however, there was a decrease in steroid levels during the night; in 2, the values approached the normal range. Data are presented on a few patients with a monosymptomatic clinical picture and normal urinary excretion of 17-ketogenic steroids. Theoretical and diagnostic implications based on the absence of variation in plasma 17-OH-CS concentration are discussed.