Purpose Patients undergoing serial cross-sectional abdominal imaging to evaluate abdominal symptomatology may have a renal tumor develop during followup of an unrelated disease process. Evaluation of such patients provides an opportunity to further define the radiographic inception, natural history and growth patterns of renal tumors. Materials and Methods Renal tumor databases from 2 institutions were reviewed for patients in whom an enhancing renal tumor developed despite a prior normal cross-sectional radiographic examination of the kidneys. Variables evaluated included age, gender, tumor size at presentation, calculated tumor growth rate from negative scan to radiographic presentation and pathology in patients undergoing definitive treatment. Results We identified 36 patients with an average age of 65 years (range 44 to 82). Mean tumor size on presentation was 2.3 cm (range 1.0 to 5.0). The presumed absolute growth rate based on the timing of the initial negative imaging study and tumor diameter at presentation was significantly greater than the observed absolute growth rate after tumor detection (0.71 vs 0.039 cm per year, p = 0.028). No difference was noted between presumed and observed tumor growth based on absolute change in tumor volume (1.44 vs 5.37 cm3 per year, p = 0.203). Presumed relative growth rates based on tumor diameter (665% vs 23% per year) and volume (1,397% vs 169% per year) were significantly greater than observed relative growth rates (p = 0.005 and p = 0.013, respectively). Conclusions The presumed growth rate of the tumors was significantly greater than the observed growth rate, suggesting that tumor growth rates do not follow a linear pattern throughout their development and progression.