The aim of this study was to evaluate the influence of regular participation in school swimming lessons on anthropometric variables and postural defect occurrence in junior high school students. An intervention group (n = 116) and a control group (n = 114) of boys and girls aged 13.4 ± 0.3 years had an equal physical education obligatory course requirement at school, with the intervention group additionally participating in swimming class once a week for 2 years. Anthropometric variables were recorded in both groups 3 times between autumn 2006 and 2008, and a clinical examination was conducted twice during this same period of time. Students from the intervention group had lower body mass than their peers in the control group at the end of the course (ANOVA, P < 0.05). The differences in body height and body mass index between the groups were insignificant. In swimming students, the correction in scoliosis was more distinct than in the control group (P < 0.05 by chi-squared test). The groups did not differ significantly in occurrence of other postural defects. Regular participation in standard swimming education at school did not have much effect on postural defects in adolescents. A special program of corrective water exercises should be applied to the school physical education syllabus.