Published reports of series of total knee replacements vary between 20% and 75% in the percentage of patients eventually having bilateral replacements. There are no data in the literature on the predicted course of the contralateral knee for patients presenting for total knee replacement. This study reviewed the history of the contralateral knee in patients with a diagnosis of osteoarthritis presenting for unilateral primary total knee arthroplasty. The current study group comprised 185 patients who had either a minimum 5 year followup (range 5-12.5 years) or who were known to have had a second knee arthroplasty before 5 years. Evaluation included a full clinical and radiographic examination of the contralateral knee at index arthroplasty and at final followup. Of the 185 patients, the contralateral side initially was asymptomatic in 36%, mildly symptomatic in 16%, moderately symptomatic in 28%, and severely symptomatic in 20%. Seventy-nine (43%) knees eventually underwent contralateral arthroplasty. Ninety-three percent of patients who had moderate or severe symptoms and severe radiographic arthritis of the contralateral side at the time of presentation later underwent total knee replacement. However, patients who presented initially with mild symptoms or who had no symptoms had only a 9% incidence of knee arthroplasty.