Several early studies suggested that spikes can be generated in the dendrites of CA1 pyramidal neurons, but their functional significance and the conditions under which they occur remain poorly understood. Here, we provide direct evidence from simultaneous dendritic and somatic patch-pipette recordings that excitatory synaptic inputs can elicit dendritic sodium spikes prior to axonal action potential initiation in hippocampal CA1 pyramidal neurons. Both the probability and amplitude of dendritic spikes depended on the previous synaptic and firing history of the cell. Moreover, some dendritic spikes occurred in the absence of somatic action potentials, indicating that their propagation to the soma and axon is unreliable. We show that dendritic spikes contribute a variable depolarization that summates with the synaptic potential and can act as a trigger for action potential initiation in the axon.