Morphometry of peroxisomes is performed on electron micrographs of ultrathin sections after staining for catalase activity with diaminobenzidine; specific peroxisomal labelling is preferred to guarantee recognition. Peroxisomal number, size, axial ratio and volume parameters are determined and compared to control values. Results from 19 patients with loss of peroxisomal functions are listed. In many patients alterations in peroxisomal morphometric features are found. A brief guideline for interpreting morphometric data is included. Diagnostically relevant morphometric alterations are summarized.