We attempted to lateralize the epileptogenic focus (seven temporal lobe hippocampal foci, one frontal lobe focus) in medically refractory unilateral complex partial seizures, using noninvasive 31P magnetic resonance spectroscopic imaging (MRSI) blindly and interictally to compare hippocampal or frontal regions. The seizure foci were more alkaline (intracellular pH = 7.17 +/- 0.03) compared with the contralateral region (7.06 +/- 0.02, p < 0.01) in all eight cases; the inorganic phosphate was relatively increased (240 +/- 50% of contralateral, seven of eight cases, p < 0.01); and phosphomonoesters were relatively reduced (68 +/- 9% of contralateral, seven of eight cases, p < 0.01). Other phosphorus metabolites were symmetric (+/- 10%). 31P MRSI correctly lateralized the seizure focus in all eight cases. By comparison, imaging correctly lateralized four cases and SPECT, two cases. In conclusion, 31P MRSI is a useful tool for the noninvasive clinical assessment of focal epilepsy and can accurately lateralize the epileptogenic focus.