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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. What this paper adds: What is already known on the subject: P300 parameters have been reported to be associated with cognitive performance in both healthy individuals and clinical groups (e.g., patients with Alzheimer's disease). Previous studies show that the presence of P300 at the early post‐stroke stage may be a predictor of better recovery of comprehension in PWA. What this paper adds to existing knowledge: Our results show for the first time that P300 may be used as a neurophysiological correlate of cognitive efficiency in PWA. In our study, P300 latency was associated with several languages and non‐language cognitive functions, especially with those whose effectiveness depends mainly on processing speed. In PWA, shorter latency corresponded to more efficient cognitive functioning. What are the potential or actual clinical implications of this work?: P300 measurement may be potentially useful in assessing the efficiency of certain cognitive functions in PWA. It may be also used to monitor the recovery process of PWA and to verify the effects of therapeutic interventions. [ABSTRACT FROM AUTHOR]