In 311 patients (160 girls and women and 151 boys and men) with lumbar spondylolysis/spondylolisthesis 30 years of age or younger at diagnosis and with at least two separate examinations available, radiographs were evaluated retrospectively to estimate the magnitude of slip progression in relation to age and to search for possible prognostic factors of progression. The mean age at diagnosis was 16.2 years, and the mean observation time was 3.8 years. Nine patients (3%) had a slip progression (defined as an increase > 20%). The magnitude of progression per year was low (0.6%). No prognostic factors for progression were found.