The present invention relates to methods of diagnosing, prognosing or treating diseases or disorders in which elevated levels of Abeta protein, including abeta.sub.1-42 are prevalent. In particular, the present invention relates to methods of diagnosing, prognosing or treating a major or minor depressive episode/disorder attributed to elevated levels of Abeta protein, including abeta.sub.1-42, found particularly in body fluids including whole blood, blood cells, serum, plasma, urine and CSF. The invention also relates to the treatment of these disorders by administering an agent that either prevents production of Abeta, prevents aggregation of Abeta fibrils, or that increases the degradation or clearance of Abeta. In addition, the invention provides a method of treating or preventing a major or minor depressive disorder comprising administering an agent that prevents or interferes with Abeta-induced neurotoxicity. The present invention also relates to pharmaceutical compositions comprising such agents and methods of screening for novel agents.