Thymidine kinase 1 (TK1) is involved in cancer progression. Metastatic site is a prognostic factor in melanoma. We assessed whether serum levels of TK1 are associated with metastatic sites or prognosis in patients treated for stage IV melanoma. The study included 64 patients, who received dacarbazine or four-drug chemotherapy (dacarbazine, vincristine, bleomycin, and lomustine) both combined with interferon-alfa. Serum samples for TK1 were analyzed by ELISA. The patients (n = 22) with only skin and subcutaneous metastases had significantly lower mean TK1 levels (1,639 pg/ml) than the patients (n = 42) with other distant metastases (2,586 pg/ml, Mann-Whitney, p = 0.031). TK1 levels above the median (1,590 pg/ml) were significantly related to deep lymph node involvement (odds ratios 3.672; 95 % confidence intervals 1.024-12.843, p = 0.036). There were no other significant associations between TK1 levels and tumor burden nor were the levels significantly related to the response to therapy or survival. Those eight patients who had received previous adjuvant IFN-alfa therapy had lower mean TK1 levels (1,735 pg/ml) as compared to those 56 patients without previous IFN-alfa therapy (2,338 pg/ml, analysis of variance, p = 0.026). This is the first study exploring serum TK1 in melanoma. TK1 might be involved in the deep lymphatic dissemination and progression of melanoma metastasis.