The authors investigated relations between obesity, age, and sex and the availabilities of striatal dopamine transporter (DAT) and extrastriatal serotonin transporter (SERT) by 123I-FP-CIT single-photon emission computed tomography. The study population consisted of 192 healthy controls with screening 123I-FP-CIT scans. Specific bindings of 123I-FP-CIT to DAT and SERT were calculated using regions of interest. Specific binding ratios (SBRs) of DAT and SERT except pons (r = 0.2217, p = 0.0026), were not correlated with body mass index (BMI). SBRs of midbrains correlated negatively with the BMIs of obese subjects (r = -0.3126, p = 0.0496), and positively with the those of non-obese subjects (r = 0.2327, p = 0.0053). SBRs of caudate nucleus (r = -0.3175, p < 0.0001), striatum (r = -0.226, p = 0.0022), and thalamus (r = -0.1978, p = 0.0074) reduced with age, and SERT availability was higher in males. However, DAT availability was similar in males and females. In conclusion, obesity has an effect on midbrain SERT availability. In addition, BMI was correlated with pontine SERT availability but not with striatal DAT availability. SERT availability was higher in males, but DAT availability showed no gender predilection.