Serum beta-glucuronidase (S beta G) activity was investigated as an early indicator of diabetes in pregnant women. The enzyme activity was determined in the sera of three groups: nonpregnant women (18 to 40 years old) with and without glucose intolerance, pregnant women with and without glucose intolerance, and pregnant women with a family history of diabetes. The S beta G activity values (means +/- SD) in normal pregnant women were 603 +/- 223, 872 +/- 270 and 1,336 +/- 353 Fishman units during the three trimesters, respectively. A slight increase in S beta G activity was found in the sera of healthy pregnant women with a family history of diabetes during the first and second trimesters. High values of S beta G (2,765 +/- 392 units) were found in all stages of pregnancy in diabetic women. Thus, the determination of S beta G activity may be a valuable aid in the early detection of reduced glucose tolerance in pregnancy.