In this retrospective and observational study, we examine grade 1 reading development following an assessment tool providing recommendations for teachers. Tests and recommendations for instruction were provided by LegiLexi, a non-profit foundation that aims to increase reading skills in primary school. We analyzed differences between poor decoders who receive a decoding recommendation after the first test session, then improve their decoding and therefore receive another recommendation (responders) and poor decoders who develop their decoding more slowly (non-responders) and receive a decoding recommendation after both the first and second assessment. Responders are characterized by initial word decoding problems but a substantial improvement between test sessions leading them to a high level of reading comprehension at the end of grade 1. Their decoding improved the most when this skill was the recommended focus of instruction but their reading comprehension also steadily improved when the recommendations changed. The group of non-responders was larger and showed improvements that were more modest on all reading tests during grade 1. The study demonstrates that decoding continues to be a bottleneck for reading development in the early school years and that recommending teachers to focus on decoding instruction might be helpful for some pupils.