Background Polycystic liver disease (PLD) is a phenotypical expression of autosomal dominant polycystic kidney disease and isolated polycystic liver disease. Somatostatin analogues, such as lanreotide, reduce polycystic liver volume. Aim To establish long-term outcome and safety of lanreotide. Methods This was an open-label, observational extension study of a 6-month, randomised, placebo-controlled trial with lanreotide (120 mg/month) in PLD. The length of total treatment was 12 months. Primary endpoint was relative change in liver volume, as determined by CT-volumetry after 12 months of treatment. We offered patients a CT scan 6 months after stopping lanreotide. Results A total of 41/54 (76%) patients participated in the extension study. Liver volume decreased by 4% (IQR -8% to -1%) after 12 months of treatment. The greatest effect was observed during the first 6 months of treatment (decrease of 4% (IQR -6% to -1%)). Liver volume remained unchanged during the following 6 months. We found that liver volume increased by 4% (IQR 06%) 6 months after end of treatment (n = 22). Conclusions Lanreotide reduces liver volume within the first 6 months of treatment and the beneficial effect is maintained in the following 6 months. Stopping results in recurrence of polycystic liver growth. This suggests that continuous use of lanreotide is needed to maintain its effect.