Objective. To study the time trends in sociodemographic and clinical characteristics of patients undergoing primary total knee arthroplasty (TKA). Methods. We used the Mayo Clinic Total Joint Registry to examine the time trends in patient demographics (body mass index [BMI] and age), underlying diagnosis, medical (Deyo-Charlson Index) and psychological comorbidity (anxiety and depression), and examination findings of primary TKA patients from 1993-2005. We used the chi-square test and analysis of variance. Results. In total, 7,229 patients constituted the primary TKA cohort; 55% were women. The mean age decreased by 1.3 years (69.3 to 68.0 years), mean BMI increased by 1.7 kg/m 2 (30.1 to 31.8 kg/m2), and mean Deyo-Charlson Index increased by 36% (1.1 to 1.5) over the 13-year study period (P ≤ 0.001 for all). Compared with 1993-1995, significantly more patients (by 2-3 times) in 2002-2005 had a BMI ≥40 kg/m2 (4.8% versus 10.6%), age <50 years (2.9% versus 5.2%), Deyo-Charlson Index of ≥3 (12% versus 22.3%), depression (4.1% versus 14.8%), and anxiety (4.1% versus 8.9%), and significantly fewer patients had an underlying diagnosis of rheumatoid/ inflammatory arthritis (6.4% versus 1.5%; P < 0.001 for all). Compared with 1993-1995, significant reductions were noted in 2002-2005 for the physical examination findings of anteroposterior knee instability, mediolateral knee instability, moderate to severe knee synovitis, severe limp, fair or poor muscle strength, and absent peripheral pulses (P ≤ 0.001 for all). Conclusion. In this large US total joint registry study, we found significant time trends in patient characteristics, diagnosis, comorbidity, and knee/extremity examination findings in primary TKA patients over 13 years. These secular trends should be taken into account when comparing outcomes over time and in policy-making decisions. Copyright © 2014 by the American College of Rheumatology.