Radiotherapy of breast cancer, cervical cancer, testicular tumours and lymphoma is one of the most effective therapy options. Damage to the nervous system, in particular the brachial and lumbar plexus, is rare and typically leads to development of progressive sensory disturbances and motor weakness after years-long latency. We present two cases exemplifying the diagnostic problems in differentiating between radiation-induced injury and recurrence of the primary tumour. A clinical course with sensorimotor symptoms and signs progressing over months, electomyographic recording of myokymic discharges, and absence of a space-occupying mass suggest late-onset radiation-induced plexopathy. The literature on pathogenesis and incidence of radiation-induced plexopathy is reviewed.