The renal function of 12 patients with non vitamin B12 responsive methylmalonic acidaemia has been investigated. Eight patients had reduced glomerular filtration rates, but the plasma creatinine concentration was only raised in those with values of less than 40 ml/min per 1.73 m2 surface area. The reduction in glomerular filtration was a function of the age and the severity of the disease. Plasma urate concentrations were increased in four patients but this may be secondary to the renal disease rather than its cause.