The effects of oral contraceptives on fasting serum lipid levels have been studied longitudinally in two groups of women. One hundred and twenty-eight subjects (group A) were tested before and during therapy; 52 subjects (group B) were tested initially during therapy and again after this had been discontinued. In both groups oral contraceptive therapy was associated with significantly raised mean serum triglyceride and cholesterol levels. No relation was found between the magnitude of change of serum triglyceride levels and the nature of oestrogen-progestogen combination, age, parity, degree of obesity, family history of diabetes, or duration of therapy. A significant elevation of the mean fasting serum triglyceride level was also found in a group of 19 women receiving low-dose glucocorticoid therapy, though the percentage increase (16%) was less than that in the women receiving oral contraceptives (49%).