Background We report on a patient with rheumatoid arthritis (RA) who was treated with adalimumab and retrospectively diagnosed as having a multiple myeloma.Investigations In addition to the determination of clinical symptoms, investigations included radiography of the thorax, spine, hands and feet, arthrosonography, determination of laboratory parameters (including C-reactive protein levels and presence of antibodies against cyclic citrullinated peptide), cytogenetics and electrocardiography.Diagnosis RA was initially diagnosed in 1988. Stage II and stage III RA were diagnosed for the left and right foot, respectively, in 1996. Joints of both hands were diagnosed with stage I RA; both wrists and some finger joints showed signs of synovitis. Plasmocytoma was diagnosed in 2004; however, investigation of medical records revealed evidence of multiple myeloma 8 years earlier, in 1996.Management RA was originally treated with gold, sulfasalazine, azathioprin and glucocorticoid. Methotrexate was later used in addition to cortisone and then in combination with a selective cyclo-oxygenase-2 inhibitor. A combination therapy consisting of adalimumab (40 mg every 2 weeks), methotrexate (15 mg weekly) and a cyclo-oxygenase-2 inhibitor (rofecoxib 25 mg daily until July 2004, etoricoxib 90 mg daily from October 2004) was started in November 2003. Adalimumab therapy was interrupted for 6 months owing to safety concerns, but was resumed after a careful risk–benefit assessment.