Objective: The aim of the present study was to explore the existence of clusters in multiple lifestyle behaviours, including physical activity (PA), sedentariness and food habits, in young adolescents. The present study also investigated whether the identified groups could be characterised by gender and components of health-related physical fitness, especially weight status and cardiorespiratory fitness (CRF). Design: A cross-sectional survey. Leisure-time PA, sedentary behaviour and food consumption were assessed by a questionnaire. Overweight prevalence was estimated using BMI (kg/m 2 ) calculated from self-reported height and weight. CRF was measured using a 20 m shuttle-run test. Setting: The study was conducted in four middle schools in Flanders, Belgium. Subjects: The sample size was 317 seventh grade students aged 11-12 years. Results: Four reliable and meaningful lifestyle clusters could be identified, labelled as 'Sporty media-oriented mixed eaters' (boys: n 34; girls: n 16), 'Academic healthy eaters' (boys: n 30; girls: n 58), 'Inactive healthy eaters' (boys: n 38; girls: n 57) and 'Inactive media-oriented unhealthy eaters' (boys: n 35; girls: n 49). The lifestyle clusters could not be characterised by adolescents' weight status. Among boys, the 'Sporty media-oriented mixed eaters' group performed significantly better on the shuttle-run test than those in clusters with the lowest levels of PA (clusters 3 and 4). Conclusions: Our results showed that healthy and risk-related behaviours may coexist in some groups of young adolescents. Isolated unhealthy behaviours, such as high levels of screen-based media use or high consumption of energydense food items, are not necessarily related with negative health outcomes, on the condition that these risk-related behaviours co-occur with more healthenhancing behaviours such as PA. Dramatic increases in childhood obesity over the past decades suggest the predominance of influential behavioural factors over genetic factors. It is generally believed that today's environment enforces an inactive lifestyle that is likely to contribute to a positive energy balance and childhood obesity (1) . Despite the important role of nutrition (energy input) and physical activity (PA; energy output) in the prevention of childhood obesity, there seems to be limited research addressing the relationship between multiple health behaviours or clusters based on this energy-balance approach (2)(3)(4) . Previous literature in this area has mainly focused on describing bivariate associations between PA and sedentary behaviour (5) , PA and food choices (6,7) or sedentary behaviour and food choices (8,9) . Although significant relationships may exist between two health behaviours, the reported correlations are predominantly weak and often too small to be of much clinical or practical significance. Nevertheless, the low correlations between health-related behaviours does not exclude the existence of clusters or groups of young people who show negative health profiles due to factors such...