This study investigates whether English-medium instruction (EMI) has an impact on Chinese undergraduates' English proficiency and affect in English learning and use. A cross-section of 136 sophomores and juniors were drawn from an English-medium and a parallel Chinese-medium program. Data included: (a) participants' scores on two national standardized English proficiency tests; (b) their English-related affect as measured by three scales adapted from Gardner's (2004) Attitude/Motivation Test Battery; (c) their perceptions of EMI in Chinese tertiary education elicited with a survey developed by the Chinese Ministry of Education (2006), and (d) interviews with 10 focal students from the English- and Chinese-medium programs. Results showed no statistically significant effect of medium of instruction on English proficiency or affect in English learning and use. However, extent of satisfaction with EMI, perceived necessity for EMI, and perceived increases in study burden had statistically significant effects on the outcome measures. Additionally, prior English proficiency was the strongest predictor of subsequent English proficiency and English-related affect. These findings raise concerns about the quality of the focal English-medium program and point to students' perceptions of EMI and prior English proficiency as crucial influences on further language learning and use.