This article reports on a study that combined two strands of research: studies of incidental vocabulary learning and studies of the classroom lexical environment (in which vocabulary exposure is measured and the potential for learning estimated). The study looked at the orthographic lexical environment in a language classroom over a semester and examined incidental vocabulary learning resulting from exposure to that environment. The investigated class, with N = 29 first language (L1)--Japanese English learners, met twice a week over a 15-week semester. Because the same courses had been taught to successive cohorts of learners for several years, it was possible to predict orthographic exposure to vocabulary in advance and thus to identify target words for investigation. In a pretest/posttest design, with the learners' actual exposure to the words monitored over the semester, word knowledge (K = 32) was gauged at two levels: word recognition and meaning recall. Significant gains were observed in each and so the influence of several factors was explored: frequency of occurrence within the class and variation in word form were found to have significant positive effects on gains, while distribution of occurrences (massed or distributed) had no effect.