To determine the effect hydration has upon the relationship between serum progesterone and its urinary metabolite pregnanediol-3 alpha-glucuronide (PDG) we measured spot samples from patients (n = 207) presenting in the 1st trimester of pregnancy. Serum and urine specimens were obtained simultaneously and measured for progesterone and PDG by radioimmunoassay (RIA). Urine specific gravity was also measured at the time of sample collection. Results demonstrated that in samples where the urine specific gravity was greater than or equal to 1.015 there was a strong positive correlation between serum progesterone and PDG (r greater than 0.60, p less than 0.001). However, when urine samples were below a specific gravity of 1.015, serum and urinary steroid values correlated poorly or not at all. We conclude that the measurement of specific gravity should be routinely performed when determining random values of urinary PDG, since only samples in which the urine is adequately concentrated accurately reflect corresponding serum progesterone concentrations.