Increased activity of brown adipose tissue (BAT) activity in adults is thought to prevent obesity. Therefore, regulators of BAT activity might serve as anti-obesity therapy in future, but are not investigated thoroughly up to now. In our study, we assessed retrospectively the association of BAT activity with several external factors and diverse chemotherapeutic and immunosuppressive agents in a collective of 702 patients. The patients underwent at least two clinically indicated PET/CT examinations in the course of different oncological and inflammatory diseases. BAT activity was identified according to predefined PET/CT criteria in all examinations. In multivariate analysis, the type of disease, the disease activity and the therapeutic regimen did not influence BAT activity. In contrast, sex and age were confirmed as independent factors for BAT activity. For the association of therapeutic agents with BAT activity, we examined 53 different disease-related agents, which were applied to patients without initial BAT activity between their PET/CT examinations. Out of these, cytarabine therapy was significantly associated with increased new onset of BAT activity. Cytarabine is a therapeutic agent for lymphoma patients. Further targeted studies might investigate the usefulness of Cytarabine serving as possible therapeutic approach against obesity via BAT regulation.