Hyperoxaluria occurs when there is a build-up of oxalate compounds in the urine. Oxalate compounds are naturally-occurring and found in foods like rhubarb. Excess oxalate in the urine can combine with calcium and form kidney stones. Recurrent kidney stones can cause scarring and loss of kidney function. With loss of kidney function, excessive oxalate will begin to build up in other organs as well, such as the bones, heart, skin and eyes. This is called systemic oxalosis.