<p>With the ever-evolving labor market and increasing demand for skilled workers, the need for continuous training has become paramount. This study aimed to assess the effectiveness of different teaching modalities – face-to-face, online, and blended – in ensuring long-term knowledge retention in adult English language learners. We compared the changes in levels demonstrated in a delayed external proficiency test and achieved in courses, controlling for student demographics, course content and volume. We analyzed the differences in levels for three student cohorts who took face-to-face (G1 = 19), online (G2 = 19), and blended (G3 = 24) courses between 2019 and 2022. We also described a flipped blended format developed for adult English courses. The results imply that a well-developed and delivered online or blended language course incorporating flipped classroom ensures at least as good knowledge retention as a face-to-face course does. This justifies recommending it to training providers. Larger samples might lead to more conclusive results.</p>