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The purpose of this study was to quantify changes in renal length, volume, and function over time after upper abdominal radiation therapy.Imaging and clinical data were retrospectively reviewed for 27 adults with abdominal radiation therapy between 2001 and 2012. All had two kidneys, radiation exposure to one kidney, and survival of at least 1 year after therapy. Mean prescribed dose was 52 ± 9 Gy to extrarenal targets. Length and volume of exposed and unexposed kidneys were measured on CT scans before treatment (baseline) and at intervals 0-3, 3-6, 6-12, 12-24, 24-36, and more than 36 months after completion of radiotherapy. Serum creatinine was correlated at each interval. Mixed-models ANOVA was used to test renal length and volume, serum creatinine, and time against multiple models to assess for temporal effects; specific time intervals were compared in pairwise manner.Mean follow-up duration was 35 months (range, 5-94 months). Exposed kidney length and volume progressively decreased from baseline throughout follow-up, with mean loss of 23% (p0.001) and 47% (p0.001), respectively. Slight increase in unexposed kidney length was not significant. Mean serum creatinine increased from 0.86 ± 0.18 mg/dL at baseline to 1.12 ± 0.27 mg/dL at 12-24 months (p0.001), then stabilized.Kidneys exposed to radiation during therapy of adjacent malignancies exhibited continuous progressive atrophy for the entire follow-up period, nearly 8 years. Volume changes were twice as great as length changes. Renal function also declined. To accurately interpret follow-up studies in cancer survivors, radiologists should be aware of the potential for progressive renal atrophy, even many years after radiation therapy.