The aim of this study was to measure progress in written English by means of an error analysis. The analysis was applied to texts produced by three different age groups and the errors found in the different error categories were used as a yardstick in the comparison of the age groups. The corpus of this study consisted of 36 compositions written by pupils in grade nine of the compulsory school and in grade one and two of the Swedish ‘gymnasium’. The major part of the study dealt with the analysis of errors in the areas of grammar, vocabulary and orthography. The results show that the percentage of orthographical errors was almost the same in all three grades. Within the group of grammatical errors there was no clear tendency found. The category of lexical errors showed a clear improvement from grade 9 through 10 while a remarkable increase of errors was found in grade 11. The study shows also that pupils make a greater improvement from grade 9 through 10 than from grade 10 through 11 with respect to the length of the compositions and the number of errors.