Schimke-immunoosseous dysplasia (SIOD) is a multisystemic disorder caused by a mutation of a putative chromatin remodelling protein. Spondyloepiphyseal dysplasia with disproportionate growth deficiency, nephrotic syndrome with focal and segmental glomerulosclerosis, defective cellular immunity, and transient ischemic attacks are major clinical features in the severe form of SIOD. In the present study we tested the hypothesis that mitochondrial dysfunction may be an underlying pathophysiologic mechanism in this multisystemic disease. Mitochondrial parameters were studied in blood (lactate, pyruvate, ketone bodies, alanine) and in urine (organic acids) of four patients with the severe form of SIOD. Activities of respiratory chain enzymes were measured spectrophotometrically in fibroblasts of two of these patients. In patients with the severe form of SIOD normal concentrations of lactate as well as normal lactate/pyruvate- and ketone-body ratios were found in plasma. Alanine, the long-term parameter for lactate, was normal as well; metabolites of the citrate cycle were not found in the urine. Activities of respiratory chain enzymes I-V were not significantly reduced in fibroblasts from two patients with the severe form of SIOD. There was no evidence for mitochondrial dysfunction in SIOD. The underlying pathophysiology of SIOD remains unclear.