A case-control study of 953 dogs with pyometra and 10,660 unmatched control dogs was used to investigate the relationships between age, parity, hormonal therapy and breed, and pyometra in Finnish dogs. Cases were reported in animals between nine months and 18 years of age, with a median age at diagnosis of nine years. Nulliparous bitches had a moderately higher risk of developing pyometra than primiparous and multiparous animals (adjusted odds ratio point estimate, 6.63). The administration of oestrogen increased the risk of pyometra in bitches up to four years of age (odds ratio point estimate > 6). No significant risk-enhancing effect of progestin treatment was detected and it is unlikely that such a risk, if present, is high (likelihood of the odds ratio exceeding 2 < 0.18). Seventeen breeds had an increased risk, and the wire-haired dachshund and mongrels had a decreased risk of developing the condition.