Objective: We aimed to study the possible factors influencing visualisation of brown adipose tissue (BAT) on FDG PET/CT at diagnosis in children. Method: A total of 92 children or adolescents were included (63 patients with Hodgkin lymphoma (HL) and 29 patients with sarcoma). We examined FDG PET/CT images obtained at diagnosis and searched for FDG uptake in BAT. We analysed the relationship between BAT activation and different possible influencing factors, such as age, sex, outside temperature or season, histological type of malignancy (HL or sarcoma), and volumetric and metabolic values of the tumour tissue. MannWhitney, Chi-2 and Fischer?s exact tests, and logistic regression analysis were used. Results: We found no significant difference in the BAT activation frequency between patients with HL and those with sarcoma (P 5 0.12). BAT visualization on FDG PET/CT in the HL subgroup showed a trend towards lower TMTV (P 5 0.06) and TLG (P 5 0.09). The logistic regression analysis did not show a significant correlation between BAT visualisation on FDG PET/CT and TMTV values. In terms of age, we found a slightly significant correlation for the whole group (P 5 0.06) and for the subgroup of HL patients (P 5 0.04) in logistic regression. In terms of seasons, the majority (45%) of BAT positive cases were found in autumn. Conclusions: BAT visualisation may be associated with a low HL tumour mass. Multivariate analysis revealed a correlation between BAT activation and age in paediatric patients. BAT visualisation do not differ between boys and girls.  Kulcsszavak: brown adipose tissue (BAT), FDG PET/CT, Hodgkin lymphoma, total lesion glycolysis (TLG), total metabolic tumour volume (TMTV), paediatric sarcoma