The aim of this study was to explore if measurement of pretransplant circulating CD161-expressing cells, in addition to clinical risk factors, could predict mucositis and infections in patients with multiple myeloma (MM) undergoing autologous stem cell transplantation (ASCT). To determine if CD161-expressing cells are likely to predict early complications, namely, mucositis (≥grade 3), infections, and cytomegalovirus (CMV) reactivation, we prospectively examined CD161-expressing cells (CD3+CD4+CD161+ and CD3+CD8+CD161+) in peripheral blood samples from 108 patients with MM undergoing ASCT. After adjusting for factors identified by univariate analysis that predicted mucositis (≥grade 3), infection before engraftment, and CMV reactivation, multivariate analyses revealed that the low proportion of CD3+CD4+CD161+ cells in peripheral blood was an independent predictor of mucositis (≥grade 3) (P=0.020), infections before engraftment (P=0.014), and CMV reactivation (P=0.010). In addition, we found that female sex and decreased glomerular filtration rate were independent factors for predicting mucositis. Female sex and severe pulmonary comorbidity were independent factors for predicting infection before engraftment. We found that the proportion of circulating CD3+CD4+CD161+ cells is useful for predicting the occurrence of early complications, including mucositis and infections, after ASCT in patients with MM.