Objective The objective of this retrospective study is to investigate the association between survival and maximum standardized uptake values (SUV max ) of liver metastases detected by pre-treatment positron emission tomography-computed tomography (PET-CT) in patients with adenocarcinoma of unknown primary origin (ACUP). Methods A total of 58 patients with ACUP and liver metastases confirmed histopathologically by liver biopsy and pre-treatment PET-CT were included in this study. SUV max values of the liver lesions were measured and their association with survival was investigated. Results The median age was 62 years; 63.8 % of the patients were males and 36.2 % were females. The median overall survival was calculated as 10.7 months (OS). The median SUV max of the liver metastases was 8.6. Accordingly, two groups were established: one with values <8.6 and the other with ones ≥8.6. No differences were detected between the two groups with respect to general characteristics. Median OS was 13.2 months in the group with SUV max <8.6 compared to 7.4 months in the group with SUV max ≥8.6. This difference was statistically significant ( p  = 0.033). SUV max (HR 1.104, 95 % CI 1.013–1.204, p  = 0.025), age (HR 1.033, 95 % CI 1.002–1.064, p  = 0.034), presence of chemotherapy (HR 2.296, 95 % CI 1.136–4.641, p  = 0.021) and LDH level (HR 1.002, 95 % CI 1.001–1.003, p  = 0.007) were identified as independent prognostic factors affecting survival in the multivariable analysis. This is the first report evaluating the impact of SUV max for liver metastases on ACUP patient survival. Conclusion The SUV max of liver metastases evaluated by PET-CT is a prognostic factor influencing survival of patients with ACUP.