Autosomal dominant polycystic liver disease (PCLD) is characterized by progressive development of multiple (>20) liver cysts. Two separate genes,PRKCSHandSEC63, have been identified to cause familial PCLD. We designed this study with two goals: to assess the relative contribution ofPRKCSHandSEC63mutations in a cohort of unrelated patients with a variable number of liver cysts, and to assess the effect of these mutations on the severity of the PCLD phenotype. We selected patients with two or more liver cysts on radiological studies and excluded those with renal cysts. A total of 51 patients entered the study and three groups were distinguished: A, 2–10 cysts (18 patients); B, 11–20 cysts (nine patients); and C, more than 20 cysts (24 patients). In total we found that eight patients with multiple liver cysts (16%) hadPRKCSH(5) orSEC63(3) mutations. Two patients (11%) from group A had missense mutations (1PRKCSHand 1SEC63). Six patients (25%) with more than 20 liver cysts had mutations (4PRKCSHand 2SEC63), of which five mutations were chain-terminating. In conclusion, bothPRKCSHandSEC63mutations are associated with polycystic liver disease. Frequency and severity of mutations is higher among patients with more than 20 liver cysts, but also patients with as few as eight liver cysts can be mutation carriers. © 2006 Wiley-Liss, Inc.