Hippocampus volumes have been shown to be decreased in patients with major depression, but volume measurements are inconsistent in patients with bipolar disorder. Both disorders are associated with deficits in hippocampus-mediated cognitive functions. However, the underlying pathophysiology is widely unknown. In this post-mortem study, we used design-based stereology on Nissl-stained serial sections to investigate the number of neurons, oligodendrocytes and astrocytes in substructures of the posterior hippocampus in eight patients with major depression, eight patients with bipolar disorder and ten control patients without a neuropsychiatric disorder. Compared to controls, patients with bipolar disorder had significantly more neurons in the cornu ammonis subfield 1 (CA1) and the subiculum, while the number of oligodendrocytes was higher only in CA1. In patients with major depression, the density of oligodendrocytes was higher in CA2/3, CA4 and the subiculum. The dose of antidepressants correlated with the density and number of oligodendrocytes in CA2/3, indicating that antidepressants may affect our results. Treatment with neuroleptics expressed in chlorpromazine equivalents and benzodiazepines expressed in diazepam equivalents correlated negatively with the number of oligodendrocytes in CA2/3 and CA4, respectively, suggesting that treatment with these drugs do not influence cell number. We did not detect alterations in either volumes of substructures or numbers of astrocytes. Increased cell numbers argue for a denser packing of neurons and oligodendrocytes as a result of a decreased neuropils. This neuropathological process may be based on neurodevelopmental disturbances and may contribute to altered microconnectivity and cognitive deficits in affective disorders.