Oral glucosamine was compared to a placebo for the management of cats with feline idiopathic cystitis (FIC) in a randomised, double-blinded, placebo-controlled, study. Forty cats with a history of recurrent cystitis due to FIC were divided into two groups and treated daily per os with either 125 mg N-acetyl glucosamine or a placebo for six months. Owners graded their cats' clinical signs at the beginning and end of the study, and kept daily diaries documenting signs of cystitis using visual analogue scales. Further episodes of cystitis were seen in 26 (65%) of the cats during the study. Affected cats experienced a mean of five recurrences (range 1-19) with each recurrence lasting a mean of four days (range 1-64 days). There were no significant differences between the two groups when considering the owners assessments of the mean health score (P>0.5), the average monthly clinical score (P=0.22) or the average number of days with clinical signs (P=0.28). Two cats suffered from such severe recurrent urethral obstruction that they were euthanased; they were both in the placebo group. Compared to the start of the study the majority of cats in both groups improved significantly (P<0.001) (mean health score of each group at the start was 0.5+/-SD 0.5, compared to glucosamine 4.4+/-0.7 and placebo 3.9+/-1.6 at the end). This is believed to have occurred because the owners of 36 of the 40 cats (90%) started feeding more canned cat food. The urine specific gravity at the start of the trial was significantly higher (mean 1.050+/-SD 1.007) than when reassessed one month later (1.036+/-1.010, P<0.01).