In 40 children with unilateral Perthes' disease, we measured the physeal slope, the angle between the physeal plane and the axis of the femoral shaft, from radiographs taken early in the disease. Thirty-seven of the 40 hips were classified as Catterall grades III and IV. Heat-at-risk signs were present in 23. We found no statistically significant difference in the physeal slope between the involved and normal hips (p = 0.20), those with or without head-at-risk signs (p = 0.96), those with or without lateral epiphyseal subluxation (p = 0.82), and different Catterall (p = 0.56) or lateral pillar (p = 0.67) gradings.