Epidemiologic findings have been inconsistent regarding the association of dietary fat, dairy products, and lactose with risk of ovarian cancer. The authors conducted a case-control study in Hawaii and Los Angeles, California, to examine several dietary hypotheses regarding the etiology of ovarian cancer in a population with a broad range of dietary intakes. A total of 558 patients with ovarian cancer diagnosed in 1993-1999 and 607 controls were interviewed regarding their diet. Consumption of all dairy products, all types of milk, and low-fat milk, but not consumption of whole milk, was significantly inversely related to the odds of ovarian cancer. Similar inverse gradients in the odds ratios were obtained for intakes of lactose and calcium, although these nutrients were highly correlated (r = 0.77). The odds ratio for ovarian cancer was 0.46 (95% confidence interval: 0.27, 0.76) among women in the highest quartile of dietary calcium intake versus the lowest (p for trend = 0.0006). The significant dietary association was limited to dairy sources of calcium (p for trend = 0.003), although a nonsignificant inverse gradient in risk was also found in relation to calcium supplement intake. These results suggest that intake of low-fat milk, calcium, or lactose may reduce the risk of ovarian cancer.