A multi-sensory structured language reading program was deployed in six first grade public school classrooms (n=150 students) in Buenos Aires, Argentina. Six control rooms usedbusiness-as-usual, whole language, instruction (n=178 students). Control and treatment roomswere well-matched in initial status. Students’ abilities and progress were recorded 3 times over27 weeks using a test battery covering basic pre-reading and reading skills: phonological awareness, decoding, vocabulary, rapid automatized naming, and executive function. At the startof 1st grade, most students have no measurable capacity for decoding. Many struggle withphonological awareness and letter naming. Very high rates of absenteeism and school closures(similar in both groups) meant that planned treatment dosage was not achieved. However, atthe endpoint of the study, students in MSL classrooms show an advantage over those in control classrooms in sentence reading comprehension, but the groups remain at parity in othermeasured skills.