1. A prospective study has been carried out, and 68 patients with hypercholesterolaemia have been investigated to study the effects of central cooling on serum lipid levels. 2. Central cooling was obtained by the exposure of the whole body to cold water. All patients were trained to gradually reduce the water temperature from 22 to 14 ;°C and to increase the time of exposure from 5 to 20 ;min over a period of 90 days. The 33 male and 35 female patients were aged between 40 and 60 years at entry with total cholesterol of 6.0 ;mmol/l or greater and low-density lipoprotein (LDL)-cholesterol of 4.0 ;mmol/l or greater. Thyroid-stimulating hormone, free thyroxine (FT 4 ), total T 3 , total cholesterol, LDL-cholesterol, high-density lipoprotein (HDL)-cholesterol, triacylglycerols and total fat mass (determined by dual-energy X-ray absorptiometry scan) were obtained at baseline and after 3 months treatment with hydrotherapy. 3. Central cooling obtained by hydrotherapy results in a median fall in tympanic temperature from 0.2 ;°C ( P < 0.001) to 0.8 ;°C ( P < 0.001). We have observed in these patients a significant reduction in total cholesterol (-0.2 ;mmol/l, P = 0.006) and LDL-cholesterol (-0.2 ;mmol/l, P = 0.004). Serum FT 4 level was higher than baseline results in 30 of these hypercholesterolaemic patients (15.5 ;pmol/l to 17.3 ;pmol/l) and there was no significant change in serum thyroid-stimulating hormone and total T 3 . 4. In conclusion, in our patients with hypercholesterolaemia we have observed a significant reduction of total cholesterol and LDL-cholesterol after body temperature regulation.