Multiple osteochondromas (MO) occur in approximately 1 in 50,000 people/yr. One in 3 patients with MO will develop valgus knee deformity (VKD), but the predictive factors for VKD are unclear. The purpose of this study was to examine the factors associated with VKD in patients with MO. From January 2003 to December 2018, 64 patients with MO visited the Nagoya University Hospital for the 1st time. Thirty-three patients with 66 limbs were sequentially included in the study after excluding 12 patients with a history of lower extremity surgery, 15 patients whose knee X-rays were unavailable, and 4 patients whose age at the last examination was <7 years. Limbs with femorotibial angle (FTA) ≥ 175° were defined as the normal group (Group N) and limbs with FTA < 175° as the valgus group (Group V), and clinical factors collected retrospectively from the medical records were compared between the 2 groups. The initial and final X-rays were compared in a subgroup analysis of 8 patients whose initial examination was <10 years old and who were followed for more than 5 years. Twenty-four males and 9 females with a median age of 17 years at the last X-rays were included in the study. The mean follow-up period was 43 ± 53 months, and the median FTA was 174.5°. Group N consisted of 32 limbs and Group V consisted of 34 limbs. Multivariate analysis was performed using the 5 factors with P-values <.15 in the univariate analysis of comparison between the 2 groups, and only medial proximal tibial angle showed significant differences (P < .001). In the subgroup analysis, multivariate analysis showed that the femoral neck-shaft angle showed significant differences between the 2 groups at the initial evaluation (P < .001). Our study suggests that medial proximal tibial angle is associated with VKD in patients with MO. Small neck-shaft angle was significantly associated with VKD, even before it became obvious. In order to study how VKD is formed, imaging of the hip and ankle joints and X-rays of the entire lower extremity should be performed in more cases.