Knee alignment and anterior cruciate ligament (ACL) injury are risk factors for knee osteoarthritis (OA). The objective was to examine interactions between knee alignment and ACL status on cartilage volume loss in participants with or at risk of knee OA. Participants were from the Osteoarthritis Initiative, a longitudinal cohort study. Data were from baseline and 24- and 72-month follow-up visits. Participants with knee OA (progression subcohort) or at risk of knee OA (incidence subcohort) that had partial or full ACL tears (OA-ACL group; n=66) or an intact ACL (OA-only group, n=367) were selected. Femur-tibia angles from radiographs quantified knee alignment. Changes in tibial and femoral cartilage volumes were measured using magnetic resonance imaging. Hierarchical linear models examined if knee alignment, presence of ACL, and their interaction were related to cartilage volume loss after accounting for other variables. Interactions between alignment and ACL status were significantly related to cartilage volume loss in the lateral plateau (β=-20.19, 95% confidence interval [CI]=-34.65 to -5.73) and lateral condyle (β=-23.64, 95%CI=-43.06 to -4.23). Valgus alignment was related to lateral compartment cartilage loss in the OA-ACL group, but not in the OA-only group. Varus alignment was related to cartilage loss in the medial plateau (β=7.49, 95%CI=0.17 to 14.80) and medial condyle (β=19.70, 95%CI=5.96 to 33.44) in both groups. The impact of knee alignment on knee OA initiation and progression varies based on ACL status. Initial lateral compartment damage or changes in joint kinematics after ACL rupture might account for these findings.Key Points• The relationship between knee alignment and lateral compartment cartilage loss depended on the status of the anterior cruciate ligament in participants with knee osteoarthritis or at risk for knee osteoarthritis.• Valgus alignment was related to lateral compartment cartilage loss in participants with a deficient anterior cruciate ligament.• Varus alignment was related to medial compartment cartilage loss regardless of the status of the anterior cruciate ligament.