This study investigated the relationship between out-of-class L2 use and proficiency gains in learners of English as a second language (ESL) in an intensive English language program. In contrast to previous studies on this topic, which have found weak, non-existent or even inverse relationships between out-of-class language experience and L2 proficiency gains, this study took place over a longer period of time (31 weeks), involved a larger number of participants (61 ESL learners from 12 different language backgrounds at four proficiency levels), and found a statistically significant connection between out-of-class language use and proficiency gains. Participants took a proficiency pre-test and post-test and responded to a questionnaire designed to elicit information about out-of-class language use. In addition, six learners participated in semi-structured interviews. Data obtained from the questionnaire and interviews were compared to gains in proficiency between the pre-test and post-test. The results corroborate the "common sense" connection between L2 out-of-class use and proficiency development. They also identify the types of out-of-class language use that are most strongly connected with L2 proficiency gains. (Contains 3 tables and 1 note.)