BackgroundPhonological processing skills have been found to contribute to spelling development across different orthographies; however, less is known about the role of orthographic knowledge. This longitudinal study explores the contribution of phonological and orthographic knowledge to spelling development in a semi-transparent orthography (Swedish) across a period of 10 years.MethodsA group of Swedish speaking children were assessed on phonological recoding (phonological choice-task), orthographic knowledge (choice-task) and spelling (dictation task) in primary school (grade 2, age 8, total N = 99), secondary school (grade 8, age 14, N = 99) and again in upper secondary school (year 2, age 17, N = 79). Furthermore, spelling in a natural writing assignment was collected in upper secondary school. Spelling scores from grade 8 (dictation) and year 2 in upper secondary school (dictation and text) were included as dependent variables in three sets of hierarchical regression analyses. In the first step spelling performance in grade 2 was included to control for the autoregressive effect. In the second step, orthographic knowledge and phonological recoding from grade 2 were entered into the model in order to test for the longitudinal prediction.ResultsTest scores within and across ages were significantly correlated in bivariate analysis. Regression analysis revealed that orthographic knowledge in grade 2 was a unique longitudinal predictor of spelling performance across time-points (secondary and upper secondary school) and assessment formats (dictation and text), beyond the contribution of the control variables.ConclusionsThis study confirms the role of early orthographic knowledge in Swedish spelling development throughout the school years assessed in standardized dictation tasks as well as in naturalistic writing assignments. What is already known about this topicImplications for theory, policy or practiceBeing able to spell well is important because it opens up the ability to communicate clearly in writing.Less is known about predictors of spelling development than of reading development.What this paper addsThis study explores the role of orthographic lexical knowledge and phonological recoding as predictors of spelling development in Swedish, which has a semi-transparent orthography.The study adds important knowledge to the field as it contains a long term follow up over a period of almost 10 years.Results show that lexical orthographic knowledge is an important longitudinal predictor of spelling development, according to both dictation and naturalistic text writing outcomes.The results have theoretical implications for understanding orthographic knowledge, and its relation to phonological processing and spelling development.The results show that already in second grade, children vary in their orthographic lexical knowledge.More research is needed in order to understand why these early differences emerged, and how to best support those falling behind in practical settings.