We describe a case of a 65-year old patient presenting with unusual mucocutaneous melanocytic proliferations of a Bilateral Diffuse Uveal Melanocytic Proliferation (BDUMP) imitating a multifocal melanoma in situ, which improved dramatically after plasmapheresis. The patient first presented at the dermatology department due to rapidly evolving brown and black macules on the glans penis. Further skin involvement of the perineal and perianal region, mamillae and oral mucosa was stated. Histology from a penile biopsy was compatible with a melanoma in situ. Due to the distribution pattern and elevated serum tumor marker S100B, metastatic melanoma was considered. Staging examinations using PET-CT scan however, revealed a lung tumor, later confirmed as a Non-small-cell lung cancer (NSCLC). Primary radio chemotherapy was initiated to treat NSCLC. Shortly after initiation of radio chemotherapy the patient developed massive vision impairment and a NSCLC-associated BDUMP was diagnosed which led to the correct classification of melanocytic skin lesions as mucocutaneous BDUMP manifestation. Plasmapheresis was started resulting in a rapid improvement of vision starting ten days after the first plasmapheresis. In contrast skin manifestations started to disappear with a marked delay 4 months after the last plasmapheresis cycle. This case highlights the importance of memorizing multiple rapidly progressing melanocytic skin and/or mucous membrane spots together with visual impairment as a possible paraneoplastic BDUMP that needs a fundamentally different therapeutic approach compared to multifocal melanoma in situ. What is already known about this topic? Bilateral Diffuse Uveal Melanocytic Proliferation (BDUMP) is a paraneoplastic syndrome with melanocytic uveal proliferation leading to vision impairment. Extraocular manifestation is rare, mainly affect the subepidermal compartment and is hard to treat. Plasmapheresis has been shown to be an effective treatment mainly for vision improvement in some but not all cases. What does this study add? Our BDUMP case with widespread skin and mucosal involvement initially mimicked a multifocal melanoma in situ and showed an excellent treatment response to plasmapheresis. Improvement of mucocutaneous lesions has not been documented well in the literature so far. We show a more than one year lasting follow up still underlining the beneficial effect of plasmapheresis in this case. In-vitro data supports the hypothesis that plasma exchange eliminates a "Cultured melanocyte elongation and proliferation (CMEP)" factor out of patient blood leading to decreased melanocyte proliferation shown numerically in-vitro and clinically in-vivo. Our case clearly indicates that before establishing a definite diagnosis and therapy in patients with rapidly evolving melanocytic skin and/or mucosal lesions BDUMP mimicking multifocal melanoma in situ should be considered making a thorough diagnostic workup mandatory.