Patellofemoral instability is a disabling condition that occurs in adolescence. Recurrence after patellar dislocation has been reported in 2-50% of patients. We compared the distal femur in patients with patellofemoral instability with distal femura of a healthy cohort using mathematical quantification of two-dimensional shape variation at the same position in different knees. One hundred eight computed tomography scans from 54 patients with patellofemoral instability were compared with 197 computed tomography scans from 102 subjects with normal knees. We used principal components analysis to quantify variation in shape of the trochlear groove as it allows comparison of knees with patellofemoral instability to healthy knees. We found that subjects who had dislocated their patella had a flattened trochlear groove, whereas normal knees had a more concave groove. The position of the trochlear groove was more medial in patients with patellofemoral instability compared with normal knees. Groove position was more important than shape when predicting which patellae were likely to dislocate.