What does it mean to have ketones in your urine?

Dehydration: It is generally a sign of dehydration. If you have diabetes, it can be a sign of diabetic ketoacidosis (too high a sugar level). It can be present in alcoholics with poor nutrition recently (alcoholic keto-acidosis) and in people who have not eaten lately (starvation keto-acidosis).

Sometimes diet based: Agree with the above answers but must add that I see many patients these days with ketones in their urine which is due to specific dietary choices. The "ketogenic" diet is a very popular diet and is similar to atkins style diets of very low carbohydrates. These diets lead to ketones in the urine.