Although the feasibility of brain‐computer interface (BCI) systems based on steady‐state visual evoked potential (SSVEP) has been extensively investigated, only a few studies have evaluated its clinical feasibility in patients with locked‐in syndrome (LIS), who are the main targets of BCI technology. The main objective of this case report was to share our experiences of SSVEP‐based BCI experiments involving five patients with LIS, thereby providing researchers with useful information that can potentially help them to design BCI experiments for patients with LIS. In our experiments, a four‐class online SSVEP‐based BCI system was implemented and applied to four of five patients repeatedly on multiple days to investigate its test‐retest reliability. In the last experiments with two of the four patients, the practical usability of our BCI system was tested using a questionnaire survey. All five patients showed clear and distinct SSVEP responses at all four fundamental stimulation frequencies (6, 6.66, 7.5, 10 Hz), and responses at harmonic frequencies were also observed in three patients. Mean classification accuracy was 76.99% (chance level = 25%). The test‐retest reliability experiments demonstrated stable performance of our BCI system over different days even when the initial experimental settings (e.g., electrode configuration, fixation time, visual angle) used in the first experiment were used without significant modifications. Our results suggest that SSVEP‐based BCI paradigms might be successfully used to implement clinically feasible BCI systems for severely paralyzed patients.