The study concerned 664 women of South-West Finland, and they were studied 5-12 weeks after delivery. The total frequency of mastitis in this population was much higher than generally reported in literature, 24% as opposed to 3%. The frequency of mastitis was similar among nulli- and multiparous women. The diagnosis was based on the judgement of midwives of physicians. If a multiparous woman has had mastitis during a previous puerperium, the probability of mastitis during a subsequent puerperium is threefold. The type of skin, its reaction of the sun, allergies, rashes, getting cold and oxytocin medication during delivery did nto affect the incidence of mastitis. Mothers under 21 and over 35 years of age had a decreased incidence (P = 0.034) of mastitis. If the women had sore nipples, the frequency increased (P = 0.003). Prophylaxis, by means of physical training, neither decreased nor increased the frequency of puerperal mastitis. The treatment advised by midwives and physicians was primarily conservative, but 38% received antibiotics; some of the antibiotics were not effective against staphylococcal infection.