The results are reported of a six-week clinical trial of the efficacy of 0.2 per cent cyclosporine ophthalmic ointment for the treatment of chronic idiopathic keratoconjunctivitis sicca in dogs in the United Kingdom, Germany and France. The 87 dogs were referral cases with a history of chronic unresponsive keratoconjunctivitis sicca of which the aetiology was unknown, and they had to meet stringent criteria before being included in the trial. The clinical response to the therapy was monitored after seven, 21 and 42 days and the results for the right and left eyes were analysed separately. There was a statistically significant increase in lacrimal secretion throughout the study, with most of the increase occurring during the first week of treatment. The percentage of eyes with improved lacrimal secretion was higher in the dogs with initial Schirmer tear test values > or = 2 mm/min than in those with initial values of 0 or 1 mm/min. The observed steady improvement in conjunctival health was not always correlated with an improvement in lacrimal secretion. The incidence of blepharospasm, other signs of discomfort and corneal oedema decreased significantly during the study. No improvement in corneal vascularisation or pigmentation was observed during the six-week trial. Overall, 76 per cent of the left eyes and 87 per cent of the right eyes were considered to have improved at the end of the treatment period. No serious adverse reactions were observed and only mild irritation was noticed by the owners immediately after the application of the ointment. This irritation resulted in the recording of an adverse reaction at the scheduled observations in only three cases.