Unconjugated hyperbilirubinemia in Crigler-Najjar (CN) disease is conventionally treated with phototherapy and phenobarbital. Orlistat treatment increases fecal fat excretion and decreases plasma unconjugated bilirubin (UCB) concentrations in Gunn rats, the animal model for CN disease. We determined in CN patients the effects of orlistat treatment on plasma UCB concentrations, and on fecal excretion of fat and UCB. A randomized, placebocontrolled, double-blind, cross-over trial was conducted in 16 patients, simultaneous with their regular treatment (phototherapy, n ϭ 11, and/or phenobarbital, n ϭ 6). Patients received orlistat or placebo, each for 4 -6 wk. Compared with placebo, orlistat increased fecal fat excretion (ϩ333%) and fecal UCB excretion (ϩ43%). Orlistat treatment significantly decreased plasma UCB concentration (-9%). In 7 of 16 patients, the decrease in plasma UCB levels was clinically relevant (Ͼ10%, mean 21%). In patients with a clinically relevant response, plasma UCB concentrations during orlistat were strongly, negatively correlated with fecal fat excretion (r ϭ -0.93). Clinically relevant response to orlistat treatment was not correlated with age, sex, CN type, BMI, or co-treatment with phototherapy or phenobarbital, but appeared correlated with a relatively lower dietary fat intake. In conclusion, orlistat treatment decreases plasma UCB concentrations, particularly in a subgroup of CN patients. Dietary fat intake may determine the responsiveness to orlistat treatment.