Parapsoriasis en plaque has been suggested to be an early manifestation of mycosis fungoides (cutaneous T-cell lymphoma). We explored the disease course of patients with small plaque or large plaque parapsoriasis in a 26-year retrospective cohort analysis of 105 parapsoriasis patients, who were clinically and histopathologically followed up in Helsinki and Tampere University Hospitals. Eventual later cancers of these patients were verified from the Finnish Cancer Registry. In the small plaque parapsoriasis group, 7 patients (10%) and in the large plaque parapsoriasis group 12 patients (35%), developed histologically confirmed mycosis fungoides during a median of 10 and 6 years, respectively. No significant differences were found regarding the risk of developing mycosis fungoides or the tendency to remission in patients treated with or without phototherapy. Our results show that not only large plaque parapsoriasis, but also small plaque parapsoriasis, as currently defined in textbooks, can progress to mycosis fungoides. The benefits of phototherapy are equivocal in parapsoriasis treatment as far as progression to cancer is concerned.