Serum levels of brain-derived neurotrophic factor (BDNF) are low in major depressive disorder (MDD), and were recently shown to decrease in chronic depression, but whether this is a trait or state marker of MDD remains unclear. We investigated whether serum BDNF levels decrease before or after the developments of MDD and other mood disorders through a case-control study nested in a cohort of 1276 women aged 75-84 years in 2008. Psychiatrists using the Structured Clinical Interview for DSM-IV identified incident cases of mood disorders at follow-up surveys in 2010 and 2012: 28 of MDDs, 39 of minor depressive disorders (minDDs) and 8 of minor depressive episodes with a history of major depressive episodes (minDEs with MDE history). A total of 106 representative non-depressed controls were also identified in the 2012 follow-up. We assayed BDNF levels in preserved sera of cases and controls at baseline and at follow-up. Serum BDNF levels at baseline in cases of MDD, minDD or minDE with MDE history were no lower than those in controls. The decrease in the serum BDNF level from baseline to follow-up was greater in cases of MDD or minDE with MDE history than in controls or cases of minDD. These results show that serum BDNF levels are not a trait marker of MDD in old women but appeared to be a state marker. The different changes in BDNF levels among diagnostic groups suggest that MDD has a pathophysiologic relation to minDE with MDE history, rather than to minDD.