The biosynthesis and efflux of sterols from cells into the medium were investigated in skin fibroblasts from a control, a patient with obligate heterozygous familial hypercholesterolaemia and a patient with the homozygous condition. The behaviour of the cells was studied in two lipid free media (lipoprotein deficient and delipidated serum), with and without the addition of low density lipoproteins (LDL) in order to find experimental conditions which showed maximum differences between the three cell lines. Incorporation of [14C]acetate into sterols in the presence (repression) and absence (induction) of LDL was similar in the normal and heterozygous cells, whereas the homozygous cells showed reduced repression and increased induction. In all three cell lines induction of sterol synthesis was greater with delipidated than lipoprotein deficient serum. The efflux of sterols in both the presence and absence of LDL did not differ between the three cell lines, but it was greater when LDL was added to the medium and when delipidated serum was used. Sterol biosynthesis and efflux from the cells of the heterozygote did not differ significantly from those of the control.