Twenty-seven adolescents with anterior knee pain with or without patellar instability were studied by computerized tomography to detect patellofemoral malalignment. The examination was performed with and without quadriceps contraction with the knee flexed to 15 degrees. Congruence angle, patellar tilt angle, sulcus angle, and trochlear depth were measured. This investigation showed the well-known types of patellofemoral malalignment with quadriceps relaxed (tilted, lateralized, lateralized, and titled patella) and, in 52% of cases, revealed changes in type and severity with quadriceps contraction (in 41%, lateralization and tilting were markedly more evident; in 11%, the type of malalignment changed). The results demonstrate that, in addition to assessment with quadriceps relaxed, computerized tomography with quadriceps contraction is a useful adjunct to diagnose and define the type of malalignment in particularly difficult circumstances.