This study examined the effect of a low-carbohydrate/high-protein diet on serum and urine ketone body concentrations. Thirteen overweight premenopausal women aged 32 to 45 years consumed ≤20 g carbohydrate/day with liberal intakes of protein and fat for 2 weeks; thereafter, carbohydrate intake increased 5 g/week for 10 weeks. Women were weighed and provided fasting urine and blood samples to detect urinary ketones and quantify serum ketone concentrations, respectively, at baseline and weeks 1 to 4, 6, and 12. Women lost 8.3%±2.8% of initial body weight by week 12. Serum β-hydroxybutyrate production was highest at week 1 and declined weekly, with all values higher than baseline (P<.05). Each week, serum β-hydroxybutyrate was correlated with presence of urinary ketones (P<.05), but no relationship was found between weekly weight change and serum ketone production. Urinary ketones are detected in premenopausal women complying with a low-carbohydrate/high-protein diet and are associated with serum ketone concentration. However, serum ketones do not reflect weight loss.