ABSTRACT The serum concentration of progesterone during pregnancy was followed in 74 diabetic women with uncomplicated pregnancies. According to the White classification the subjects included class B, C and D patients. It is shown by comparison with a reference group of normal pregnant women, that diabetics in the late part of pregnancy have normal or high levels of serum progesterone, and a reference interval for uncomplicated, diabetic pregnancies is computed. The progesterone level in serum is correlated to the pregnanediol excretion in urine and a significant correlation (2α < 0.05) is found between the progesterone concentration (nmol/l serum) and the pregnanediol concentration (μmol/l urine). In contrast, no significance is seen, when the progesterone concentration is correlated to the amount of pregnanediol (μmol/24 h) excreted during the day. A highly significant correlation (2α < 0.001) is shown between the serum concentrations of progesterone and placental lactogenic hormone (HPL).