The study developed an adaptive computer-assisted reading system and investigated its effect on promoting English as a foreign language learner-readers' contextual vocabulary learning performance. Seventy Taiwanese college students were assigned to two reading groups. Participants in the customised reading group read online English texts, each of which was customised by the developed system to offer immediate and repeated meetings with previously encountered unknown words, while participants in the typical reading group read online texts without control on unknown word recurrence. After the 4-week online reading treatment, the two groups of learners were tested by online immediate and delayed vocabulary tests, and the students from the customised reading group were also required to complete the questionnaire regarding system use. The results showed that the vocabulary mean scores from both immediate and delayed testing demonstrated significantly better results in word gain and word retention with the customised reading group and that the adaptive reading system was appealing for the students. The study demonstrated that by providing customised reading with word recurrence specific to individuals' unknown words, this adaptive computer-assisted English as a foreign language reading system creates a more favourable condition for foreign language vocabulary growth.