Starting the first grade of primary school in the Republic of Croatia, students begin learning how to read and write within their formal education. One of the main tasks primary school teachers have is to ensure that their students master how to read and write the Croatian Latin script (Mendeš, 2009). During the first grade, students master how to write uppercase and lowercase block and cursive letters (Odluka 2006), which is why it is expected that students know how to write all four variations at the end of the first grade. The aim of this research is to describe the acquisition of writing uppercase block letters in first-grade primary-school students at the end of the school year, as well as to detect the most common deviations in their handwriting performance. The research included 72 first-grade students from two primary schools in Zagreb. The data were collected by using two tasks – letter naming of uppercase and lowercase block letters, and writing down words in uppercase block letters from dictation. To assess the acquisition of writing letters when given as the initial letters of dictated words, criteria from the PredČiP triage test (Kuvač Kraljević & Lenček, 2012) were applied. The same criteria were used to define the performance of writing uppercase block letters. The obtained results indicate that children achieve the highest results according to “Detail” and “Line execution” criteria, and have the poorest performance when it comes to “Spacing” and “Closure”. Furthermore, this study also suggests that the most demanding letters for students are those that consist of a combination of straight and curved lines. Research results do not show any difference in the overall performance between sexes. However, a more detailed analysis shows that girls perform better in writing letters that consist of a straight line and a combination of straight lines. The differences in the overall result regarding hand dominance were not found, but it is important to consider the great disproportion in the number of left-handed and right-handed students that may have affected the accuracy of the obtained results for this difference. Finally, students who named more uppercase and lowercase block letters also achieved a higher overall result in writing uppercase block letters. All obtained data describe the acquisition of writing uppercase block letters among first-grade primary-school students at the end of the school year. The results of this study show an uneven acquisition of writing letters and point to the need for more systematic guidelines in teaching initial writing.