The invention relates to an improved method for predicting the onset of, diagnosing, prognosticating and/or treating dementing diseases. The method comprises determining the level of heme oxygenase-1 suppressor (HOS) activity and/or factor in tissue or body fluid obtained from a patient, and comparing said level with the corresponding level of HOS activity and/or factor in corresponding tissue or body fluid obtained from at least one control person. The tissue or body fluid is suitably blood, plasma, lymphocytes, cerebrospinal fluid, urine, saliva, epithelia or fibroblasts. The method is useful where the dementing disease is any of Alzheimer Disease, Age-Associated Cognitive Decline, Mild Cognitive Impairment, Parkinson disease with dementia, Progressive Supranuclear Palsy, Vascular (i.e. multi-infarct) Dementia, Lewy Body Dementia, Huntington's Disease, Down's syndrome, normal pressure hydrocephalus, corticobasal ganglionic degeneration, multisystem atrophy, head trauma, neurosyphilis, Creutzfeld-Jacob disease and other prion diseases, HIV and other encephalitides, and metabolic disorders such as hypothyroidism and vitamin B12 deficiency. The method may also prove useful in differentiating the "pseudodementia" of depression from Alzheimer disease.