Aims The aim was to gain insight into the biology of primary cutaneous CD4+ small/medium T‐cell lymphoproliferative disorder (PCSM‐LPD). Methods We describe the histopathological and clinical characteristics of 177 PCSM‐LPD diagnosed at our consultation centre. We performed immunohistochemical multistaining in a subset of cases (n = 46) including PD1, Cyclin D1, and multiple markers of proliferation. We evaluated clonal T‐cell‐receptor‐(TCR) rearrangements and used tissue microdissection to analyse TCR‐clonality of PD1(+) cells. Results The cohort of n = 177 PCSM‐LPD included 84 males and 93 females (median age 57, range 13–85). Clinical presentation was as a solitary nodule or plaque (head and neck > trunk > extremities). Most patients were treated by local excision or steroids (96%, 69/72); relapses occurred in 12/65 (18%) of patients with follow up. Histopathology revealed the predominance of a nodular pattern (75%, 134/177) and frequent clustering of PD1(+) large cells (70%, 103/147). We detected Cyclin D1 and PD1 coexpression (>10% of PD1(+)‐cells) in 26/46 (57%), which was not associated with CCND1 breaks or amplifications. PD1(+)‐cells in PCSM‐LPDs showed a significantly higher expression of proliferation‐associated proteins compared to PD1(−)‐cells. A clonal TCR‐rearrangement was present in 176/177 (99%), with a clonal persistence in 7/8 patients at relapse including distant sites. Tissue‐microdissection revealed PD1(+)‐cells as the source of clonality, whilst PD1(−)‐cells remained polyclonal. Conclusion PCSM‐LPD is a clinically indolent, albeit neoplastic, disease driven by clonal expansion of PD1(+)‐cells. We demonstrate Cyclin D1‐expression associated with accelerated proliferation as a surprising new biological feature of the disease.