Although pronunciation is an integral part of speaking, the role pronunciation plays in determining speaking-proficiency levels is unclear (Higgs & Clifford, 1982; Kang, 2013). To contribute to the understanding of this area, the research reported here investigated the relationship between English as a Second Language (ESL) learners’ pronunciation ability and their speaking-proficiency ratings. At an intensive English program (IEP) in the United States, a speaking test was administered to 223 ESL students. Their speaking proficiency was rated using an oral proficiency assessment based on standardized guidelines. In addition, their pronunciation was rated in six categories (vowels, consonants, word stress, sentence stress, intonation, and rhythm) by 11 raters using a rubric specifically developed and validated for this study. Many-Facet Rasch Measurement (MFRM) was used to calculate the students’ pronunciation ability, which was then compared to their speaking ability. The study found that sentence stress, rhythm, and intonation accounted for 41% of the variance in the speaking-proficiency test scores with sentence stress being the most powerful factor. Keywords: ESL, speaking proficiency, English pronunciation, pronunciation rubric, assessment, sentence stress, suprasegmentals