Objective: To establish whether there is a relationship between nutrition knowledge and dietary behaviour in members of commercial slimming clubs. Design: A self-completed questionnaire on nutrition knowledge, dietary behaviour and factors associated with weight-loss management. Setting: Members of slimming clubs in their home environment. Subjects: The targeted sample comprised of 56 members of slimming clubs in the Greater Manchester area recruited through social media and the local Rosemary Conley slimming club leader. Results: The level of nutrition knowledge in the study population was high, however this was not significantly correlated with dietary behaviour which was poor (r=0.054; p=.694). Similarly, nutrition knowledge and dietary behaviour were not significantly correlated with education levels, age or alcohol consumption (p>.05). However, a significant inverse relationship was found between educational attainment and BMI (r=-0.392, p=.005). Barriers to weight loss were not reported to be major factors by this study group; the greatest benefits of membership were support and encouragement from other members and the club leader. The proportion of returning members was >70%. Conclusions: Dietary behaviour in members of commercial slimming clubs is not significantly influenced by nutrition knowledge. Although healthy eating recommendations can be valuable, other factors are more important for achieving weight loss, particularly support and fellowship from other members. Slimming club members regain the weight lost after leaving the clubs and inevitably re-join.