BACKGROUND: In view of recent reports of the relationship of kidney disease to birth weight, we evaluate the relationship between birth weight and chronic kidney disease (CKD), including end-stage kidney disease, in Australian adults. STUDY DESIGN: A case-control study. SETTING & PARTICIPANTS: Patients attending the nephrology department at a major metropolitan hospital in Australia were asked to recall their birth weight, excluding those with structural kidney abnormalities. Two controls for each patient, matched for sex and within 5 years of age, were selected from participants from the Australian Diabetes, Obesity and Lifestyle (AusDiab) Study, who had also been asked to report their birth weight. PREDICTOR: Birth weight in kilograms. OUTCOMES & MEASUREMENTS: CKD and stages were defined using the National Kidney Foundation-Kidney Disease Outcomes Quality Initiative classification, proteinuria as a marker of kidney damage, and glomerular filtration rate estimates, by using the Modification of Diet in Renal Disease Study equation. RESULTS: Of 189 patients with CKD who reported their birth weights for whom controls were identified, 106 were men. Mean age was 60.3 +/- 15 (SD) years. Mean birth weight overall was 3.27 +/- 0.6 versus 3.46 +/- 0.6 kg for their controls (P < 0.001), and proportions with birth weights less than 2.5 kg were 12.2% and 4.4% (P < 0.001). In patients with CKD, 22.8%, 21.7%, 18%, and 37.6% were in CKD stages 2 (n = 43), 3 (n = 41), 4 (n = 34), and 5 (n = 71), respectively. Birth weights by CKD stage and their AusDiab controls were as follows: stage 2, 3.38 +/- 0.52 versus 3.49 +/- 0.52 kg; P = 0.2; stage 3, 3.28 +/- 0.54 versus 3.44 +/- 0.54 kg; P = 0.1; stage 4, 3.19 +/- 0.72 versus 3.43 +/- 0.56 kg; P = 0.1; and stage 5, 3.09 +/- 0.65 versus 3.47 +/- 0.67 kg; P < 0.001. Differences in birth weights applied to women and men and people younger than 60 and 60 years and older and were present in the major "causal" categories of renal disease. LIMITATIONS: Birth weight is by self-recall with a significant nonresponse rate to the questionnaire in both cases and controls. CONCLUSIONS: Urban Australian patients with CKD had lower birth weights than their matched Australian controls. In addition, the more advanced the CKD stage, the lower the birth weight. Thus, lower birth weights appear to predispose to CKD and to its progression. Among possible explanations is the documented association between birth weight and nephron number.