This study aimed to determine the relationships between the amplitude and cortical-activating areas in each hemisphere for 14-Hz centro-parietal spindles to clarify the involvement of the cortex in the asymmetry of spindles after hemispheric stroke with putaminal or thalamic hemorrhage using simultaneous recording of the electroencephalogram and magnetoencephalogram. Spontaneous cortical activities during sleep stage 2 (spindles) were simultaneously recorded from 10 patients with putaminal or thalamic hemorrhage with a 60-channel electroencephalogram and a 306-channel whole-head magnetoencephalogram. Based on the frequencies and cortical distributions recorded with electroencephalogram, the 14-Hz centro-parietal and 12-Hz frontal spindles were differentiated. The cortical sources of the 14-Hz centro-parietal spindles were identified with magnetoencephalogram as equivalent current dipoles. Activation centered in four areas, the pre- and postcentral areas in posterior frontal cortex and parietal cortex of each hemisphere. However, these areas were not always activated simultaneously. Reduction of amplitude in the affected hemisphere after hemispheric stroke indicated reduction of areas of activation or activation strengths in the ipsilateral hemisphere. For some spindles, when reduction of amplitude of above 50% occurred in the affected hemisphere compared with the nonaffected hemisphere, no activation occurred in the ipsilateral hemisphere. Reduction of amplitude was related to the reduction of areas of cortical activation and activation strengths. Reduction of amplitude also occurred over the nonaffected hemisphere, and reduction of cortical activating area and activation occurred in the nonaffected hemisphere, suggesting that the cerebral hemispheres are involved in the eneration of spontaneous sleep spindles. The stronger ipsilateral effects of cerebral lesions on spindle oscillations indicated reduction of amplitude of sleep spindles in the ipsilateral hemisphere, as well as reduction of cortical activation of spindle oscillations and underlying corticothalamic projections. Simultaneous electroencephalogram and magnetoencephalogram recordings can yield valuable findings concerning the functional asymmetry between hemispheres after hemispheric stroke from the relationships between the amplitude and patterns of cortical activation associated with spindles.