Focal segmental glomerulosclerosis is scar tissue that forms in parts of the kidney called glomeruli. The glomeruli serve as filters, helping rid the body of unnecessary or harmful substances. Each kidney has thousands of glomeruli. One glomeruli is called a glomerulus.

"Focal" means that some of the glomeruli become scarred, while others remain normal. "Segmental" means that only part of an individual glomerulus is damaged.

The cause of focal segmental glomerulosclerosis is usually unknown. A small number of cases result from reflux nephropathy. The condition affects both children and adults. Men and boys are affected slightly more often than women and girls, and it also occurs more frequently in African-Americans. There are also rare, inherited forms of focal segmental glomerulosclerosis.

Focal segmental glomerulosclerosis causes about 10 - 15% of all cases of nephrotic syndrome.