The aim was to find the differences in ketogenesis initiation in the early period after the exercise in obese patients and to find if these changes may predict the weight loss during the physical activity program. 96 females were enrolled. A clamped heart rate test (CHR) was performed to establish comparable exercise intensity. Blood samples for beta hydroxybutyrate (BOHB) assessment were collected prior, immediately after and 60 min after the test. Patients underwent a three month fitness program. Anthropometric measurements (fat mass and biochemical parameters) were measured. An energy intake was monitored and comparable in all subjects. A significant increase of BOHB was found in 60(th) minute after the test, when compared with initiation levels (BOHB1 vs. BOHB3; p=0.03). This increase correlates with % fat mass (R=0.196; p=0.02) and negatively with age (R= -0.147; p=0.05) and with weight reduction during the three-month program (R= -0.299; p=0.03). Serum BOHB increase after the single exercise may detect individuals with an ability to induce lipolysis in three-month program of physical activity for obese patients.