The authors obtained venous blood samples from 106 female residents of western Washington in whom serous and mucinous borderline ovarian tumors had been diagnosed between 1980 and 1985 ("cases") and from a randomly selected telephone sample of 134 control women from the same counties. Mean activity in plasma of alpha-L-fucosidase was slightly higher in cases (334 units/ml) compared to controls (306 units/ml). The risk for a serous borderline tumor was virtually the same for women in all four quartiles of alpha-L-fucosidase activity. Women in the three lowest quartiles of activity were at 50-70% the risk of women in the highest quartile for the development of a mucinous tumor, but within these three quartiles there was no trend of decreasing risk with decreasing activity. These results suggest that for most women plasma alpha-L-fucosidase activity has little or no bearing on the risk of borderline ovarian tumors. Whether such an association exists in women genetically predisposed to these tumors could not be assessed.