Ever since Robert Atkins, MD, popularized his high-protein, high-fat, low-carbohydrate diet in the 1970s, the mainstream medical community has predicted disastrous consequences, from increased risk of colon cancer (because of low fiber) to heart disease. In the past year, however, surprising results of the first serious research efforts on the diet have trickled in, suggesting it brings about weight loss without apparent harm. But before you dive headlong into a juicy steak and béarnaise sauce, know that the findings are—at best—both preliminary and ambiguous. "My largest worry," says Gary Foster, PhD, who headed one of the most talked about studies, "is that the public will misinterpret this data as license to eat a high-saturated-fat diet." Researchers are holding out for the more definitive conclusions of a much larger, five-year National Institutes of Health study that is just getting off the ground and will monitor effects on cholesterol and bone, along with weight loss. Here's what scientists know today about the safety and effectiveness of the Atkins diet-and what they don't.