Evidence has been accumulating which supports a role for the cerebellum in motor learning. Motor learning is though to be mediated by complex spikes acting as an error signal, which when firing in conjunction with simple spike activity modify synapses between parallel fibers and Purkinje cells. We studied the activity of neurons in the posterior lateral cerebellar cortex of macaques that were performing reaches to visual targets. We found that simple spike firing in many of these neurons was modulated by whether the monkey successfully hit the target or not. The success-failure modulation was present for reaches using either arm and could persist for several hundred milliseconds into a period when the monkey was constrained from moving its arms. This temporally extended success-failure activity could interact with complex spike firing in order to enhance learning, particularly when the motor command is temporally separated from sensory feedback.