Single-photon emission computed tomography (SPECT) using metaiodobenzylguanidine (MIBG) is an important diagnostic tool for the treatment of refractory pheochromocytoma and paraganglioma (PPGL). Owing to the difficulty of SPECT quantification, the tumour-to-background ratio (TBR) is used to assess disease activity. However, the utility of TBR is limited owing to the background setting. A quantification technique of SPECT/computed tomography (CT) would facilitate image interpretation. This study aimed to assess the relationship between 123I-MIBG maximum standardized uptake value (SUVmax) and TBR and levels of urinary catecholamines and metabolites in patients with refractory PPGL.This study included 15 patients with refractory PPGL who underwent 131I-MIBG therapy. Overall, 27 123I-MIBG SPECT/CT images were acquired before and after the therapy. Lesions observed on whole-body images were analysed; the maximum number of lesions per scan was 10. 123I-MIBG SUVmax was semi-automatically calculated using Q. Metrix package (GE Healthcare). TBR was manually calculated according to the following formula: (max count in lesion - max count in background)/max count in background. Background was set in the contralateral area. When a background region of interest could not be set in the area, it was set in the thigh area. Urine was sampled for 24 h to measure catecholamine and metabolite levels. Increases of ≥3-fold were considered abnormal. TBR, 123I-MIBG SUVmax and urinary catecholamine and metabolite levels were compared using linear regression analysis.All patients had MIBG-avid lesions, as seen on 123I-MIBG SPECT/CT. A significant relationship between 123I-MIBG SUVmax and TBR was observed (correlation coefficient [r] =0.84, P < 0.0001). In 27 SPECT/CT examinations, normetanephrine (NMN) level was abnormally increased in 51% (14/27), but other catecholamine and other metabolites were abnormally increased in < 26% (7/27). 123I-MIBG SUVmax strongly correlated with NMN (r=0.76, P < 0.01) and log NMN (r=0.74, P < 0.01).123I-MIBG SUVmax demonstrated similar trends as TBR and reflected urinary NMN in patients with refractory PPGL. Semi-automatic quantification of SPECT/CT could be a useful tool for the evaluation of disease activity.