The steroid hormone 17-OH Progesterone (17-OHP) is produced in the adrenal cortex and in the gonads. This hormone is of clinical significance because it represents the ultimate precursor of 11β-desoxycortisol (compounds, CpS). CpS is formed by hydroxylation of the carbon atom C 21. Enzyme activity of 21-hydroxylase in the adrenal cortex may be monitored by analyzing the level of 17-OHP in the blood. Deficiencies in 21-hydroxylase, commonly found in congenital adrenal hyperplasia, result in excessive secretion of 17-OHP and consequently increased in blood levels. Deficiencies in 11-hydroxlase, however, merely lead to moderately increased values of 17-OHP. The analysis of 17-OHP plays a significant role in the differential diagnosis of Congenital Adrenal Hyperplasia (CAH). In adult non-pregnant women, 17-OHP levels in the blood depend on the phase of the menstrual cycle. Like progesterone, 17-OHP is secreted by the mature follicle and the corpus luteum. Concentrations are generally higher after ovulation. In addition, levels of 17-OHP are influenced by daytime rhythms which correlate with the adrenal secretion of cortisol. Maximal levels are found in samples collected between midnight and 8.00 a.m. In adult men, there are few indications of similar fluctuations of 17-OHP levels.