Studies have suggested that dysfunctional beliefs about cancer have a significant influence on psychological well-being. However, few studies have focused on specific psychological symptoms. The goal of this study was to assess the predictive capacity of dysfunctional beliefs about cancer at baseline on levels of depression, anxiety, insomnia and fatigue symptoms over an 18-month period. Patients scheduled to undergo surgery for cancer (N = 962) completed a questionnaire assessing cancer beliefs at baseline (T1) and the Hospital Anxiety and Depression Scale (HADS), the Multidimensional Fatigue Inventory (MFI) and the Insomnia Severity Index (ISI) at T1, as well as 2 (T2), 6 (T3), 10 (T4), 14 (T5) and 18 (T6) months later. At T1, participants were categorized into four groups, based on the severity of their dysfunctional beliefs about cancer (none, mild, moderate or severe). Group x time factorial analyses using mixed models and an adjusted alpha level of 0.006 revealed that the four symptoms tended to decrease throughout the 18-month period, but participants endorsing higher levels of dysfunctional beliefs about cancer consistently reported more severe symptoms. Patients with higher levels of dysfunctional beliefs about cancer had significantly: (a) higher anxiety scores throughout the 18-month period; (b) higher depression scores until the 14-month assessment; (c) higher fatigue scores until the 6-month time point; and (d) greater sleep difficulties at baseline and at the 6-month assessment. Baseline dysfunctional beliefs about cancer appear to predict the severity and persistence of psychological symptoms, especially depressive and anxiety symptoms. These results highlight the relevance of integrating cognitive therapy early during the cancer care trajectory, in order to prevent the persistence of psychological disturbances over time.