Summary. High serum Mucin-1 (sMUC-1) levels have been shown in patients with adenocarcinoma and multiple myeloma (MM). We evaluated sMUC-1 levels in 76 patients with MM, 6 with plasma cells leukaemia (PCL) and 89 with monoclonal gammopathy of undetermined significance, to establish prevalence data and verify its possible prognostic role. Of the 171 patients, 27 [16%; 95% confidence interval (CI): 10–21%] had high sMUC-1 levels compared with healthy subjects (1·5%; 95% CI: 0–4%). Elevated sMUC-1 levels in MM and PCL patients correlated with anaemia and elevated serum lactate dehydrogenase levels; these patients showed a shorter survival than those with normal sMUC-1 levels (median overall survival: 25 vs. 49 months, P = 0·003).