The diagnosis of mycosis fungoides (MF) is difficult in early stages and is based on a combination of clinical findings and histopathologic criteria. The aim of this study was to assess the diagnostic delay in MF and to investigate the rationale for multiple biopsies in a single-centre, retrospective study of 157 patients with MF. The first biopsy was diagnostic for MF in 25% of cases. The median diagnostic delay was 2.3 years and depended on whether the diagnosis was established after one or multiple biopsies. The chance of a biopsy resulting in a diagnosis of MF was 25% irrespective of the number of the biopsy in the sequence. There was a significant diagnostic delay, especially in patients in whom the initial biopsy was not specific. Sampling error and unnecessary postponement of subsequent biopsies are likely factors and therefore multiple biopsies should be considered in patients with skin lesions suggesting MF.