Decreased heart iodine-123 metaiodobenzylguanidine ((123)I-MIBG) uptake [heart-to-mediastinum count ratio (H/M)] is reported in heart disease (HD) or Lewy body disease (LBD). When LBD is merged, therefore, information regarding HD severity may be ambiguous. We aimed to examine whether lung (123)I-MIBG uptake [lung-to-mediastinum count ratio (L/M)] assessment might be useful for differentiating two clinical conditions of HD and LBD, and to investigate whether L/M could reflect the grade of left ventricular (LV) dysfunction.Three groups were examined: LBD (patient group with Parkinson's disease or dementia with Lewy bodies, n = 33), PS (group with other Parkinsonian syndromes, n = 20) and HD (group with heart disease). HD consisted of 4 subgroups: HD(I) [H/M(<2.30)-matched group with LBD, n = 34), HD(II) [H/M(≥2.30)-matched group with PS, n = 33], HD(III) [group for functional analysis, LV ejection fraction, first-third and peak filling rates (1/3FR and PFR) and time to PFR were calculated using gated SPECT, n = 35] and HD(IV) (group for examining cardiac prognosis, follow-up period of 1283 ± 506 days, n = 54). Using Doppler echocardiography, a diastolic parameter E/e' and pulmonary artery pressure (ePAP) were estimated.H/Ms did not differ between HD(I) and LBD, or between PS and HD(II). However, L/Ms were increased in the order of LBD, PS, HD(II) and HD(I) groups. In combined LBD, PS, HD(I) and HD(II), L/Ms correlated positively with a diastolic parameter E/e'. L/Ms correlated with ePAP, while H/Ms did not. H/Ms correlated with a systolic parameter EF (r = 0.56) and diastolic parameters 1/3FR (r = 0.51) and PFR (r = 0.51), and L/Ms correlated with diastolic parameters 1/3FR (r = -0.36) and PFR (r = -0.36) but not with EF in HD(III). Kaplan-Meier analysis showed earlier cardiac death in patients with decreased H/Ms, but not in patients with increased L/Ms in HD(IV).Our study suggest that increased lung (123)I-MIBG uptake is useful as a reference marker for differentiating two clinical conditions of HD and LBD, and can reflect the degree of LV diastolic dysfunction. Elevated ePAP caused by LV diastolic dysfunction may be involved in the mechanism(s) of increased lung uptake.