One hundred sixty skin biopsy specimens from 89 patients with the clinical diagnosis of large plaque parapsoriasis and 240 specimens from 106 patients with mycosis fungoides were reviewed. Through the use of chart reviews and a retrospective questionnaire, various factors (sex, age, history of eczema/atopy, occupation) were examined in these two patient groups. Mycosis fungoides developed in 30% of the patients in the parapsoriasis group. Nineteen percent of patients in the mycosis fungoides group had worked in industry. Once the clinical diagnosis of mycosis fungoides was considered, an average of four biopsy specimens were needed to establish the diagnosis. The average interval from the initial visit to the diagnosis of mycosis fungoides from examination of biopsy specimens was 22 months. These findings support further the view that large plaque parapsoriasis represents an important precursor of mycosis fungoides. A designation of premycosis fungoides would emphasize this relation more than the term parapsoriasis.