['To quantify the environmental component of aetiology of overweight and obesity by examining the relationship between the degree of overweight in dogs and cats and the degree of overweight in their owners.', 'Cross-sectional study. Main outcome measures of the owners were weight, height (stature) and BMI. Of the animals, weight and divergence from ideal weight were measured by a veterinarian.', 'Three veterinary clinics in Amsterdam, The Netherlands.', 'Dogs and cats, together with their owners, who visited the veterinary clinic. Dogs and cats had to be older than 1 year, and their owners had to be at least 21 years old. After exclusion, there remained forty-seven pairs of dogs and their owners and thirty-six pairs of cats and their owners.', 'We found a significant relationship between the degree of overweight of dogs and the BMI of their owners (r = 0.31). Correction for length of ownership, gender and age of the animal, and gender, age, education level and activity score of the owner did not materially affect this relationship. However, after correction for the amount of time the dog was being walked each day, this relationship disappeared. No significant relationship was found between the degree of overweight of cats and the BMI of their owners.'] Is overweight in dogs , but not in cats , related to overweight in their owners?