The efficacy and tolerability of clomipramine in the treatment of separation anxiety in dogs was tested in a prospective, randomized, double-blind, placebo-controlled, parallel-group, international multicenter clinical trial. For a diagnosis of separation anxiety, dogs had to exhibit at least one of the following signs in the absence of their owner: destruction, defecation, urination and/or vocalization, as well as the behaviour suggestive of "hyper-attachment" to their owner. A total of 95 dogs were randomized to receive one of the three treatments for 2-3 months: "standard-dose" clomipramine (1 to <2 mg/kg, PO, q. 12 h); "low-dose" clomipramine (0.5 to <1 mg/kg, PO, q. 12 h); and placebo (PO, q. 12 h). All dogs received behavioural therapy. Dogs were examined at four time points (days 0, 28, 56 and 84) after the initiation of therapy. Improvement in each dog's behaviour at days 28, 56 and 84 was evaluated in comparison to its behaviour at day 0.The results showed that, compared to placebo, dogs receiving standard-dose clomipramine were rated improved at least three times faster for the signs destruction, defecation and urination. At most time points, more dogs in the standard-dose clomipramine group were rated improved for the signs destruction, defecation and urination, and in an owner's global assessment of the dog's overall behaviour (p<0.05 at certain time points). However, there were no statistically significant differences at any time point between the standard dose and the placebo groups in the sign vocalization. The low-dose clomipramine group produced no statistically significant effect when compared with placebo. Mild and transient vomiting was noted as a side effect of clomipramine in a small number of dogs.It is concluded that addition of standard-dose (1 to <2 mg/kg, PO, q. 12 h) clomipramine to conventional behavioural therapy for 2-3 months ameliorated the signs of separation anxiety in dogs.