Background: Aphasia is often accompanied by impairment of non-language cognitive functions. Assessment of cognitive capacity in people with aphasia (PWA) with standard neuropsychological methods may be problematic due to their language difficulties. Numerous experimental studies indicate that P300 may be considered as an index of cognitive capacity in both healthy and clinical samples. Accordingly, the measurement of event-related potentials enables the investigation of behaviourally non-observable mental processes underlying the cognitive functions that are assessed with neuropsychological tests. Aims: To investigate in PWA the relationship between P300 parameters and cognitive function efficiency measured with neuropsychological methods. Methods & Procedures: A total of 25 PWA after left-hemispheric stroke participated in the study. Electrophysiological (EEG) signals were recorded during the performance of a visual Go-No Go task. P300 was identified on nine electrodes, which were then pooled in three lines: left (F3, C3, P3), central (Fz, Cz, Pz) and right (F4, C4, P4). The neuropsychological assessment of cognitive functions included mental speed, short-term memory, divided attention, executive functions, auditory language comprehension and expression. Outcomes & Results: P300 latency correlated with indices of several cognitive functions: temporal resolution, psychomotor speed, spatial short-term memory, planning, word and sentence comprehension, as well as verbal fluency. Shorter P300 latencies were accompanied by greater efficiency of the abovementioned functions. In contrast, significant correlations between P300 amplitudes and cognitive measures were fragmentary. Conclusions & Implications: In PWA, P300 latency might be related to cognitive functioning, especially to measures that rely heavily on the speed of information processing. However, P300 seems to be unrelated to more complex cognitive functions. P300 latency may be used as a neurophysiological correlate of cognitive efficiency in PWA and might have potential applications in monitoring the effects of therapeutic interventions in this patient group.