PURPOSE: The relationship of proximal tibial morphology to the presence of femoral osteochondritis dissecans (OCD) lesions is unknown. This radiographic study tested the null hypothesis that knees with unilateral medial or lateral OCD lesions would have no difference in the slope of their medial, lateral, or posterior tibial plateau compared with unaffected knees. METHODS: There were 72 patients with unilateral OCD lesions of the medial or lateral femoral condyle seen at our institution from 2005 to 2011. On AP and lateral radiographs of the knee, three examiners conducted independent measurements of the tibial plateau posterior slope, as well as medial and lateral slope as measured from the peak of the tibial spine to the edge of the plateau on the side of the corresponding OCD lesion. Measurements were repeated on normal contralateral and matched control knees. RESULTS: Knees with medial femoral condyle OCD lesions had greater medial tibial slope compared with normal contralateral knees (p = 0.007) and normal controls (p < 0.04). Knees with lateral femoral condyle OCD lesions had no significant difference in lateral tibial slope compared with the contralateral knee or matched controls. Posterior slope was greater in knees with medial OCD lesions than matched controls (p = 0.007). Intraclass correlation coefficients demonstrated consistency between observers for all measurements. CONCLUSION: An assessment of proximal tibial morphology demonstrated greater medial and posterior tibial slope in knees with medial OCD lesions compared with normal knees. The technique for measuring medial and lateral tibial slope was reliable among evaluators. The clinical relevance is that proximal tibial morphology may have a relationship with OCD lesions. LEVEL OF EVIDENCE: III.