LDH may represent an indirect marker of neo-angiogenesis and worse prognosis in many tumour types. We assessed the correlation between LDH and clinical outcome for biliary tract cancer (BTC) patients treated with first-line chemotherapy. Overall, 114 advanced BTC patients treated with first-line gemcitabine and cisplatin were included. Patients were divided into two groups (low vs. high LDH), according to pre-treatment LDH values. Patients were also classified according to pre- and post-treatment variation in LDH serum levels (increased vs. decreased). Median progression free survival (PFS) was 5.0 and 2.6 months respectively in patients with low and high pre-treatment LDH levels (p = 0.0042, HR = 0.56, 95% CI: 0.37-0.87). Median overall survival (OS) was 7.7 and 5.6 months (low vs. high LDH) (p = 0.324, HR = 0.81, 95% CI: 0.54-1.24). DCR was 71% vs. 43% (low vs. high LDH) (p = 0.002). In 38 patients with decreased LDH values after treatment, PFS and OS were respectively 6.2 and 12.1 months, whereas in 76 patients with post-treatment increased LDH levels, PFS and OS were respectively 3.0 and 5.1 months (PFS: p = 0.0009; HR = 0.49; 95% IC: 0.33-0.74; OS: p < 0.0001; HR = 0.42; 95% IC: 0.27-0.63). Our data seem to suggest that LDH serum level may predict clinical outcome in BTC patients receiving first-line chemotherapy.