Lithium-induced polyuria, although common, often goes unrecognized. The purpose of the present study was therefore to investigate the complaints of polyuria, and distress and functional impairment associated with polyuria, in 56 patients with bipolar disorder on long-term lithium treatment. All participants underwent 24-h urine collection, and renal function tests. Polyuria (24-h urine volume > 3 L) was found in 70% of subjects. Unless directly enquired about, polyuria was underreported. Impairment in work and daily routine due to increased urine output/frequency was associated with 24-h urine volumes. Polyuria is a highly prevalent, distressing and impairing side-effect of long-term lithium treatment, requiring due attention.