This grounded theory study explores conceptualisations of chronic fatigue syndrome/myalgic encephalomyelitis from semi-structured interviews with 10 health-care professionals working with children and adolescents. The findings suggest that a lack of a clear empirical understanding of chronic fatigue syndrome/myalgic encephalomyelitis leads to ‘working with uncertainty’, whereby health-care professionals utilise previous experiences to make sense of the condition and inform their clinical practice. How health-care professionals make sense of chronic fatigue syndrome/myalgic encephalomyelitis may influence the labels given to young people and the interventions they receive. The findings provide insight into a currently understudied area, and highlight potential avenues for further research and clinical practice.