OBJECTIVE Study lower urinary dysfunction in familial amyloidotic polyneuropathy (FAP). METHODS Fifty-four FAP patients were studied. Clinical examination, urodynamics and ultrasound of the urinary tract were performed. RESULTS Urinary symptoms appeared during the first three years of the disease in 50% of the patients. The initial urinary symptom was dysuria in 39% and incontinence in 24% of the patients, sensitivity and contractility disturbances of the detrusor were found at the initial stages. Non-relaxing urethral sphincter was found in 51,7% and dyssynergia in 37,5% of the cases. Ultrasound revealed thickening of the vesical wall in 42,5% of the patients, more common in males (M:16; F:7). Opening of the vesical neck was found in 56% of the cases (M:19; F:11) with paradoxical closure during the attempt to void. Fluctuations in the opening of the vesical neck were found in eight patients, also more frequently in males (M:6; F:2). CONCLUSIONS In addition to reduced sensation, underactive detrusor, opening of the vesical neck and external sphincter deficit, we found data suggesting failure of relaxation of the internal and external sphincter. The overdistention associated with an open vesical neck and external sphincter deficit justifies incontinence in those patients. The retention is due to inadequate contraction of the detrusor, probably associated with non-relaxing of the internal and external sphincter. These dysfunctions derive from deposition of amyloid substance in the detrusor, but overdistention is likely to play a role. Early therapeutic intervention in these patients is vital to avoid secondary injuries.