Objective The purpose of this study was to present the characteristics and outcome of patients with proven pheochromocytoma or paraganglioma who had false-negative 123I-MIBG SPECT. Methods Twenty one patients with false-negative 123I-MIBG SPECT, (6 males, 15 females) aged 13–55 years (mean 40.9 years) were included. We classified them according to the stage of the disease as non-metastatic or metastatic at the time of false-negative 123I-MIBG SPECT study, the location and size of the tumor, plasma and urinary catecholamine and metanephrine levels, genetic mutations, and outcome in terms of occurrence and progression of metastases and death. Results Thirteen patients were evaluated for metastatic tumors while 8 others were seen for non-metastatic disease. All primary tumors and multiple metastatic foci did not show avid 123I-MIBG uptake regardless of the tumor diameter. The majority of patients had extra-adrenal tumors with hypersecretion of normetanephrine or norepinephrine. SDHB mutation was present in 52% (n=11) of cases, RET mutation in 4% (n=1), and the rest were apparently sporadic. Twenty four percent (n=5) had metastatic disease on initial presentation. Fourteen patients were followed-up for 3–7 years. From them, 71% (n=10) had metastatic disease and majority had SDHB mutation. Nine are still alive while 5 (4 were SDHB) died due to metastatic disease. Conclusion A false-negative 123I-MIBG SPECT is frequently related to metastatic tumors and usually due to SDHB mutations with unfavourable prognosis. We, therefore, recommend that patients with false-negative 123I-MIBG SPECT be tested for SDHB mutations and to undergo more regular and close follow-up.

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