BACKGROUND: To investigate the clinical significance of interictal serum prolactin level (ISPL), especially in the medical intractability, in patients with complex partial seizure involving the temporal lobe. METHODS: Forty-one male patients older than 16 years, who had motionless staring, were selected. The patients were divided into paired groups according to medical intractability, secondary generalization, duration of disease, seizure frequency, number of antiepileptic drugs, hippocampal sclerosis in MRI, and interictal epileptiform discharge. Serum was sampled in the morning before breakfast, and ISPL was measured. RESULTS: ISPL of the medically intractable patients (11.43 +/- 1.25 ng/ml, n=23) was relatively higher than the treatable patients (9.67 +/- 1.52, n=18), but it was not statistically significant (p>0.05). ISPL of the patients with secondary generalization was significantly higher (11.58 +/- 1.10, n=33) than that of the patients without secondary generalization (6.84 +/- 1.39, n=8; p=0.049). The other factors did not make significant changes in ISPL (p>0.05). CONCLUSIONS: The medical intractability of epilepsy with clinically determined complex partial seizures involving temporal lobe did not change the ISPL. Therefore, ISPL may not be useful as a biochemical marker of the medical intractability of complex partial seizures involving the temporal lobe. However, there was the evidence suggesting that the secondary generalization of complex partial seizures may be an important factor to increase ISPL, which implies that more widespread robust activation of limbic structures may be needed to elevate ISPL.