A frequently cited concern of very-low-carbohydrate diets is the potential for increased risk of renal disease associated with a higher protein intake. However, to date, no well-controlled randomized studies have evaluated the long-term effects of very-low-carbohydrate diets on renal function. To study this issue, renal function was assessed in 68 men and women with abdominal obesity (age 51.5±7.7 years, body mass index [calculated as kg/m2] 33.6±4.0) without preexisting renal dysfunction who were randomized to consume either an energy-restricted (∼1,433 to 1,672 kcal/day), planned isocaloric very-low-carbohydrate (4% total energy as carbohydrate [14 g], 35% protein [124 g], 61% fat [99 g]), or high-carbohydrate diet (46% total energy as carbohydrate [162 g], 24% protein [85 g], 30% fat [49 g]) for 1 year. Body weight, serum creatinine, estimated glomerular filtration rate and urinary albumin excretion were assessed before and after 1 year (April 2006-July 2007). Repeated measures analysis of variance was conducted. Weight loss was similar in both groups (very-low-carbohydrate: -14.5±9.7 kg, high-carbohydrate: -11.6±7.3 kg; P=0.16). By 1 year, there were no changes in either group in serum creatinine levels (very-low-carbohydrate: 72.4±15.1 to 71.3±13.8 μmol/L, high-carbohydrate: 78.0±16.0 to 77.2±13.2 μmol/L; P=0.93 time × diet effect) or estimated glomerular filtration rate (very-low-carbohydrate: 90.0±17.0 to 91.2±17.8 mL/min/1.73 m2, high-carbohydrate: 83.8±13.8 to 83.6±11.8 mL/min/1.73 m2; P=0.53 time×diet effect). All but one participant was classified as having normoalbuminuria at baseline, and for these participants, urinary albumin excretion values remained in the normoalbuminuria range at 1 year. One participant in high-carbohydrate had microalbuminuria (41.8 μg/min) at baseline, which decreased to a value of 3.1 μg/min (classified as normoalbuminuria) at 1 year. This study provides preliminary evidence that long-term weight loss with a very-low-carbohydrate diet does not adversely affect renal function compared with a high-carbohydrate diet in obese individuals with normal renal function.