A 32-year-old man who was receiving adalimumab for seronegative rheumatoid arthritis presented with a 4-week history of fever, night sweats, fatigue, myalgias and diarrhoea. On examination, he had obvious splenomegaly but no lymphadenopathy or pharyngitis. Full blood count revealed mild neutropenia and significant lymphocytosis, with a blood film showing atypical lymphocytes. Liver function tests were mildly deranged with a mixed hepatitic and obstructive pattern. Ultrasound confirmed massive splenomegaly with a span of 21 cm in the long axis. Serological tests confirmed the presence of both primary Epstein-Barr virus and cytomegalovirus infections. The patient had his adalimumab withheld, was treated with supportive measures and improved over a period of 8 weeks. He remained well 5 months after the onset of illness with complete normalisation of blood count and a resolution of the splenomegaly.