Estrogen metabolism in women with SLE is weighted towards 16alpha-hydroxyestrone, an estrogenic compound that might fuel disease activity. Indole-3-carbinol (I3C) is a nutritional compound that can shift estrogen metabolism towards less estrogenic metabolites. However, the effects of I3C in women with SLE have not been studied. Open-label 1-week metabolic study of 375 mg/day I3C was carried out in women with SLE, followed by a 3-month observational period for disease activity. The primary outcome measure was the change in ratio of urinary 2:16alpha hydroxyestrone levels. Secondary measures included the SLE Disease Activity Index. Seventeen clinically premenopausal women fulfilling ACR criteria for probable/definite SLE (mean age 37.9 y, range 20-49 y, mean disease duration 4.3 y, range 0.5-15) completed the 1-week metabolic study; 12 took I3C for 3 months. The mean 2:16alpha hydroxyestrone ratio increased by 1.84 to 3.15 (P = 0.0001). Mean SLEDAI scores were 10.0 (baseline); 6.25 (3 months); and 8.8 (3 months after withdrawal; P = NS). Women with SLE can manifest a metabolic response to I3C and might benefit from its antiestrogenic effects. We did not observe any striking effects on SLE disease activity during the 3-month observational period.