Background Tibiofemoral alignment has a role in knee osteoarthritis (OA), but which factors contribute to alignment is unknown. Objective To investigate familial aggregation of tibiofemoral alignment in participants of the GARP (Genetics ARthrosis and Progression) study. Methods The tibiofemoral anatomical angle on semiflexed knee radiographs was measured in sibling pairs (mean age 60 years, 81% women) with primary OA with multiple joint involvement. Radiographic OA was assessed according to the Kellgren-Lawrence (KL) method. Heritability estimates of the tibiofemoral angle were calculated by comparing twice the between-sibling variance with the total variance; adjustments were made for age, gender, body mass index, history of meniscectomy, lower limb fracture and in analyses including all knees, for KL score. Results 360 subjects representing 180 families were studied. The mean (SD) tibiofemoral angle of right and left knees in the probands was 182.7 (2.9)degrees and 182.8 (2.6)degrees, respectively; similar angles were measured in the siblings. Radiographic knee OA (KL score >= 2) was present in 27% of the knees. Stratified analyses in sib pairs with non-osteoarthritic right or left knees showed adjusted heritability estimates of the tibiofemoral angle of the right and left knees of 0.42 (95% CI 0.02 to 0.82) and 0.56 (95% CI 0.19 to 0.93). In addition, adjusted heritability estimates of the tibiofemoral angle in all right and left knees were calculated, being 0.48 (95% CI 0.18 to 0.78) and 0.50 (95% CI 0.21 to 0.79), respectively. Conclusion The alignment of the tibiofemoral joint is influenced by familial factors, implying that tibiofemoral malalignment may add to the genetic predisposition for knee OA development. These results need to be confirmed in other study populations.