To evaluate the presumed peroxisomal involvement in cholesterol/isoprenoid biosynthesis, we determined the protein levels and activities of five different enzymes of the presqualene segment of the cholesterol/isoprenoid biosynthetic pathway in primary skin fibroblasts of selected patients with a peroxisomal biogenesis disorder (PBD). These five enzymes all have been reported to be partly or exclusively peroxisomal and include HMG-CoA reductase, mevalonate kinase, phosphomevalonate kinase, mevalonate pyrophosphate decarboxylase, and isopentenyl pyrophosphate isomerase. To exclude that genetic differences, resulting in different defects in peroxisomal biogenesis, have differential effects on the activity of the cholesterol biosynthetic enzymes and on de novo cholesterol biosynthesis, we chose fibroblasts of patients with defined defects in one of four different PEX genes leading to Zellweger syndrome (PEX1, PEX5, PEX16 or PEX19). We found that all enzymes measured are at least as active in the peroxisome-deficient cells cultured in cholesterol-depleted medium as in identically cultured control cells. This indicates that if these presumed peroxisomal proteins are mislocalized to the cytosol they do not loose their activity, nor get degraded unlike most other authentic peroxisomal proteins. We also measured de novo cholesterol synthesis from radio-labeled acetate in all cell lines and found similar or even elevated rates for the PBD cells when compared to controls. Our results imply that functional peroxisomes are not a prerequisite for the functioning of enzymes involved in cholesterol/isoprenoid biosynthesis and as such raise doubts about the true involvement of peroxisomes therein.