Peripheral, predominantly motor polyneuropathy is associated with a plethora of possible aetiologies and the investigative procedures to rule them out are extensive. A 31 year old lady presented with progressive lower limb weakness over a period of 2 years. Examination revealed symmetrical weakness (3+/5) of all distal muscle groups in the lower limbs with absent ankle jerk. There was no sensory loss. The nerve conduction study revealed a predominantly axonal motor neuropathy. Cerebrospinal fluid examination and other investigations to rule out potential causes of predominantly motor peripheral neuropathy were normal. The only positive findings were multiple qualitative assays for porphobilinogen in her urine.