The objective of the study was to estimate the prevalence of overweight and obesity in children and adolescents in the Basque Country, Spain. It consisted of an analysis of baseline data of the Nutrition Survey, a cross-sectional study, carried out in 2004–2005. The study population comprised child and adolescent living in the Basque Country. The analysis was carried out in a representative random sample of 1178 people aged 4–18 years. Anthropometric examinations were undertaken by trained observers using standardized methods and included measurements of weight and height. Subjects were classified into different body mass index categories, according to the International Obesity Task Force guidelines. A 5.4% of the population studied was obese; 6% of males and 4.7% of females, the highest in the 11–14 age group in boys (7.2%) and in the 4–6 age group in girls (12.5%). Overweight (22.9%) was slightly higher in girls. The highest prevalence of excess weight (overweight+obesity) was observed in girls aged 4–6 years (38.4%), decreasing with age. Subjects in the 15–18 age group rated 16.6%. Boys evidenced a higher excess weight rate in the 11–14 (32.9%) and 7–10 (32%) age groups; the lowest rate was found in the 4–6 age group. Prevalence of obesity was higher in the less privileged socio-economic strata (6.9% vs. 5.2%), for both boys and girls. However, this trend was observed only in girls for overweight (25.9% vs. 21.8%). This study shows a high prevalence of obesity and overweight in the studied population and similar to other European countries and regions.