We report clinical findings in 15 patients with lymphomatoid papulosis (LyP) associated with mycosis fungoides (MF). LyP either preceded (n = 4), followed (n = 5) or occurred concurrently with the MF lesions (n = 6). Twenty-eight LyP lesions were classified histologically and analysed further with immunostaining for CD3 and CD30. Five biopsies contained a predominance of type A cells, six biopsies contained a predominance of type B cells. and six were mixed (A + B). However, 11 biopsies contained a population of atypical mononuclear cells with large hyperchromatic nuclei that we have termed indeterminate cells. These cells contained a thin rim of eosinophilic cytoplasm and showed strong CD30 but absent, faint or normal CD3 staining. In seven biopsies from five separate patients these cells represented the predominant cell type and we have termed this the pleomorphic variant of LyP. Analysis of T-cell receptor genes using Southern blot analysis and polymerase chain reaction/single strand conformational polymorphism analysis identified a T-cell clone in six of 16 LyP lesions and nine of 16 MF lesions. In the three patients who had clones in both types of skin lesions, the clones were identical. Only two of 10 blood samples, both of which were from the same patient, had a T-cell clone and none of two lymph nodes showed evidence of a clonal population. To date all patients are alive with a median follow-up of 15 years from the onset of the first lesion. One patient has developed Lin anaplastic large cell lymphoma of the nasopharynx. These data augment the current literature on the association of LyP and MF and suggest that the