Oestriol, a naturally occurring short-acting oestrogen, was used to treat acquired urinary incontinence in 129 bitches selected by 48 veterinary practitioners in the Netherlands, Belgium, France and Germany. The dogs were treated daily for 42 days with oestriol tablets, using a self-controlled study design. The dogs were examined and blood sampled at the beginning and end of the trial. According to the veterinary practitioners 83 per cent of the dogs either became continent or improved, but the others showed no change or became worse. The owners reported similar results: 82 per cent of the dogs responded to treatment and the others did not. The dose and treatment schedule for each dog were established on the basis of clinical efficacy. Mild and transient oestrogenic effects such as swelling of the vulva and attractiveness to male dogs were observed soon after the treatment began and at the higher dose schedule used in 12 of the dogs. A haematological examination of 114 of the dogs revealed no abnormalities.