This study aimed to investigate whether cranberry extract could reduce lower urinary tract (LUT) and gastro-intestinal (GI) signs in feline idiopathic cystitis (FIC). Twenty-one client-owned cats were randomly allocated to two groups: a treated group (T, n = 10) receiving daily an oral nutritional supplement containing cranberry extract and a control group (C, n = 11). Owners were trained to recognise daily LUT and GI signs. Physical examination, urinalysis and bladder ultrasonography were performed at day 0 (T0), 15 (T15), 30 (T30), 60 (T60). Both groups showed an improvement for dysuria and periuria from T0 to T30 (p < 0.05), but only in cats of the T group, LUT signs disappeared at T60. A significant improvement in the T group was also observed for GI signs and bladder ultrasonography at T60 (p = 0.03). Urinalysis did not show any significant differences. This preliminary study suggests that cranberry could be effective in reducing LUT and GI signs in FIC