To determine the effects of lifestyle intervention and diet on body composition, anthropometric measurements, and metabolic syndrome (MS) in obese and overweight adolescents and their mothers, a diet and lifestyle intervention program was administered for 16 weeks to 19 9-17-year-old (12.52 ± 2.85 years) adolescents (female/male, 8/11) with a body mass index (BMI) value over the 90th percentile; hemoglobin (Hb)A1C, fasting insulin, homeostasis model assessment of insulin resistance (HOMA-IR), fibrinogen, and C-reactive protein levels of the adolescents and anthropometric measurements of the mothers were compared. In some of the anthropometric values (body weight (BW), BMI, waist circumference (WC), skinfold thicknesses, body fat tissue, and lean tissue mass), a statistically significant difference was observed in pre- and post-application measurements (p<0.05). There was a significant difference in pre- and post-application values of systolic and diastolic blood pressure and some biochemical parameters (uric acid, total cholesterol, high density lipoprotein, low density lipoprotein, triglyceride, and HOMA-IR) (p<0.05); however, there was no significant difference in pre- and post-application values of fasting blood glucose, aspartate transaminase, alanine transaminase, albumin, HbA1C, fasting insulin, and fibrinogen (p>0.05). MS was observed in 52.6% of the participants at baseline, and this rate was found as 15.8% based on the measurements carried out at week 16, which is a statistically significant decrease (p<0.05). There was a statistically significant difference in pre- and post-study values of BW, BMI, body fat mass, basal metabolic rate, WC, hip circumference, hip/waist, and skinfold thickness in mothers. A 16-week diet and lifestyle intervention program for overweight and obese adolescents involving their mothers resulted in significant improvement in obesity and MS treatment.