Deep brain stimulation (DBS) is an adjustable, reversible, nondestructive neurosurgical intervention using implanted electrodes to deliver electrical pulses to areas in the brain. DBS has recently shown promising results as an experimental treatment of refractory obsessive–compulsive disorder (OCD). The novelty of the treatment requires careful observation of symptoms and possible side effects in patients. This case report describes two patients with treatment-refractory OCD in whom increased voltage of deep brain stimulation targeted at the nucleus accumbens increased impulsivity. Voltage increase of stimulation resulted in an immediate inflation of self-confidence, irritability and impulsive behaviour and was reversed only after lowering the voltage. The mechanisms behind DBS are not yet fully understood. Possibly, stimulation in the area of the nucleus accumbens affects the corticostriatal circuitry, which plays an important role in impulsivity. Location and amplitude of stimulation might be critical in inducing these behaviours. These two cases underline the importance of a careful clinical assessment of impulsive behaviours during DBS for OCD.