In human temporal lobe epilepsy, a loss of hilar neurons followed by the sprouting of recurrent mossy fiber collaterals and the reinnervation of free synaptic sites on granule cell dendrites are discussed as possible mechanisms underlying hippocampal hyperexcitability. Dentate granule cells have been shown to upregulate brain-derived neurotrophic factor (BDNF) as well as TrkB, the high-affinity receptor for BDNF, in response to limbic seizures. This raised the possibility that BDNF is an important factor in hippocampal mossy fiber sprouting. Here we have used slice cultures of hippocampus, in which mossy fibers sprout and form a supragranular plexus in response to granule cell deafferentation, and have compared cultures from early postnatal BDNF-deficient mice and wild-type mice. We demonstrate that there is sprouting of supragranular mossy fibers in cultured slices from both BDNF knock-out and wild-type mice. We conclude that BDNF is not an essential factor for mossy fiber sprouting. However, our data do not exclude a role for BDNF in mossy fiber sprouting in wild-type mice, as compensatory mechanisms might have become effective in the mutant.