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Aims/hypothesis: Oxidative stress is involved in the pathogenesis of diabetic nephropathy. The antioxidant enzyme catalase plays a key role in redox regulation in the kidney. We investigated associations of catalase gene (CAT) polymorphisms and plasma catalase activity with diabetic nephropathy in type 1 diabetic patients., Methods: We genotyped nine single nucleotide polymorphisms (SNPs) in the CAT region in participants from the Survival Genetic Nephropathy (SURGENE) (340 French participants, 10 year follow-up) and the Génétique de la Néphropathie Diabétique (GENEDIAB) (444 Belgian and French participants, 8 year follow-up) study cohorts. Replication was performed in a Brazilian cross-sectional cohort (n = 451). Baseline plasma catalase activity was measured in SURGENE (n = 120) and GENEDIAB (n = 391) participants., Results: The A allele of rs7947841 was associated with the prevalence of incipient (OR 2.79, 95% CI 1.21, 6.24, p = 0.01) and established or advanced nephropathy (OR 5.72, 95% CI 1.62, 22.03, p = 0.007), and with the incidence of renal events, which were defined as new cases of microalbuminuria or progression to a more severe stage of nephropathy during follow-up (HR 1.82, 95% CI 1.13, 2.81, p = 0.01) in SURGENE participants. The same risk allele was associated with incipient nephropathy (OR 3.13, 95% CI 1.42, 7.24, p = 0.004) and with the incidence of end-stage renal disease (ESRD) (HR 2.11, 95% CI 1.23, 3.60, p = 0.008) in GENEDIAB participants. In both cohorts, the risk allele was associated with lower catalase activity. Associations with incipient and established or advanced nephropathy were confirmed in the replication cohort., Conclusions/interpretation: CAT variants were associated with the prevalence and incidence of diabetic nephropathy and ESRD in type 1 diabetic patients. Our results confirm the protective role of catalase against oxidative stress in the kidney.