Mycosis fungoides is the most prevalent form of primary cutaneous T‐cell lymphoma. Patients frequently present with early‐stage disease typically associated with a favourable prognosis and survival of 10–35 years, but over 25% may progress to advanced disease with a median survival < 4 years, and just 13 months in those with nodal involvement. Sézary syndrome presents in advanced disease with erythroderma, blood involvement and lymphadenopathy. The Bunn and Lamberg staging system (1979) includes stages IA–IIA (early‐stage disease) and IIB–IVB (advanced‐stage disease) and provides prognostic information, but some patients with tumour‐stage disease (IIB) have a worse prognosis than those with erythrodermic‐stage (III). Conversely, patients with plaque‐stage (IB) folliculotropic mycosis fungoides may have a worse outcome than those with tumour‐stage (IIB). The more recent staging system of the European Organisation for the Research and Treatment of Cancer/International Society for Cutaneous Lymphoma has been designed to reflect tumour burden at different sites. However, this staging system has not been validated prospectively for prognosis. Furthermore, this staging system does not include a detailed measurement of skin tumour burden, as indicated by the modified skin weighted severity assessment tool. This assessment measures body surface area of disease and is weighted to record patch, plaque and tumour to produce a numerical value from 0·5 to 400 and is an established endpoint for clinical studies. Nor does this staging include clinicopathological features associated with a poor prognosis such as folliculotropism. Here we review the clinical, haematological, pathological and genotypic parameters outside the staging system, which may affect survival in mycosis fungoides and Sézary syndrome. Most studies are retrospective and single centre. The identification of poor prognostic factors may be used to develop a prognostic index to use alongside staging, which may be of benefit in mycosis fungoides/Sézary syndrome to identify patients with a potentially poor prognosis.