The aim of this study was to investigate muscle size satisfaction and predisposition to health harmful muscle gain practice in bodybuilders and recreational gymnasium users and to evaluate its relationship with sport mastery. The sample consisted of 132 men (51 bodybuilders and 81 recreational gymnasium users). The muscle size satisfaction and related variables were evaluated using a 23-item questionnaire (alpha=0.6, test-retest reliability 0.7). The difference between the actual and the desired girths of the widest site of the upper arm and the thigh was determined based on the subjects' reported data. The study showed that the majority of the gymnasium users (61.2%) wanted to gain muscle mass, but the tendency was observed that muscle size dissatisfaction, preoccupation with body shape, obligatory motivation, and obsessive attitude towards exercising depended on the sport mastery - the lower mastery predicted higher values. Entering bodybuilding competitions was associated with a 3.2-time (95% CI 1.14-9) higher muscle size dissatisfaction and a 4.8-fold increase (95% CI 1.4-16) in reported predisposition for health harmful muscle gain practice; however, being a bodybuilder was associated with 5.7-time (95% CI 1.9-17) higher predisposition for a health harmful practice. In conclusion, bodybuilders have lower muscle size satisfaction and significantly higher predisposition to health harmful muscle gain practice as compared to recreational gymnasium users. Our findings service for understanding that competitive bodybuilders are not body image homogeneous group if their sport mastery is ignored.