A considerable body of research has investigated the effectiveness of extensive reading on incidental vocabulary acquisition in second language (L2) learners. However, we still know very little about the relationship between extensive reading and vocabulary development among Saudi learners of English as a foreign language (EFL) where language classroom is the predominant source of learning, if not the only one. In the present case study, a single participant was instructed to read extensively for eight weeks as an informal activity outside the classroom. The participant's written receptive vocabulary knowledge was measured before and after the treatment. Results indicated that extensive reading contributed largely to the participant's vocabulary gain, suggesting that a vocabulary uptake of about eight words from extensive reading intervention has occurred compared to about two words per contact hour from language classroom input where reading texts are short and scattered throughout the textbook. Finding is interpreted in order to provide some pedagogical recommendations.