Cafestol and kahweol–diterpenes present in unfiltered coffee— strongly raise serum VLDL and LDL cholesterol and slightly reduce HDL cholesterol in humans. The mechanism of action is unknown. We determined whether the coffee diterpenes may affect lipoprotein metabolism via effects on lipid transfer proteins and lecithin:cholesterol acyltransferase in a randomized, double-blind cross-over study with 10 healthy male volunteers. Either cafestol (61–64 mg/day) or a mixture of cafestol (60 mg/day) and kahweol (48–54 mg/day) was given for 28 days. Serum activity levels of cholesterylester transfer protein, phospholipid transfer protein and lecithin:cholesterol acyltransferase were measured using exogenous substrate assays. Relative to baseline values, cafestol raised the mean (±S.D.) activity of cholesterylester transfer protein by 18±12% and of phospholipid transfer protein by 21±14% (both P<0.001). Relative to cafestol alone, kahweol had no significant additional effects. Lecithin:cholesterol acyltransferase activity was reduced by 11±12% by cafestol plus kahweol (P=0.02). It is concluded that the effects of coffee diterpenes on plasma lipoproteins may be connected with changes in serum activity levels of lipid transfer proteins.