OBJECTIVE: To assess the concurrent validity of alternative measures of frontal plane knee alignment, namely the radiographic anatomic axis and 5 clinical measures, in medial compartment knee osteoarthritis (OA) as compared with the mechanical axis on radiograph. METHODS: Forty individuals (mean +/- SD age 64.7 +/- 9.4 years) with symptomatic medial knee OA participated. Knee alignment was measured according to the following methods: lower-limb mechanical axis on radiograph, lower-limb anatomic axis on radiograph, visual observation, distance between medial knee joint lines or medial malleoli using a caliper, distance between a plumb line and medial knee joint line or malleolus using a caliper, tibial alignment using a gravity inclinometer, and lower-limb alignment using a goniometer. Data were analyzed using Pearson's correlation coefficient or Spearman's rho correlation coefficient and simple linear regression. RESULTS: The anatomic axis best correlated with the mechanical axis (r = 0.88), followed closely by the inclinometer method (r = 0.80). Other clinical measures of alignment that were significantly associated with the mechanical axis were the caliper method, the plumb-line method, and visual observation (r = 0.76, 0.71, and -0.52, respectively). However, the goniometer method failed to correlate. CONCLUSION: The anatomic axis on radiograph and the inclinometer method appear to be valid alternatives to the mechanical axis on full-leg radiograph for determining frontal plane knee alignment in medial knee OA. These alternative methods of measuring knee alignment may increase the assessment of this parameter by clinicians and researchers alike, given that malalignment is an important indicator of disease progression and treatment outcome.