For a period of 22 months, postpartial women in Oslo were asked to consult one of several specific general practitioners in the event of mastitis. Clinical symptoms, bacteriological findings in breast milk and treatment were recorded in 43 patients. Patients with a favourable (n = 35) and with an unfavourable outcome (n = 8) defined as abscess, relapse and/or relief of symptoms after more than seven days, were compared. Unfavourable outcome was characterized by higher score of clinical symptoms and a higher isolation frequency of Staphylococcus aureus. The occurrence of fever did not differ between the groups. Bacteriological findings in milk from both breasts were compared with the findings from 100 milk donors. Staphylococcus aureus was more frequently isolated in milk from affected breasts than from unaffected and control breasts (17/40 versus 4/40 versus 4/100). Most of the Staphylococcus aureus strains (70%) were betalactamase producers. Coagulase negative staphylococci were a frequent finding in all milk samples, whereas Gram-negative bacteria were frequent only in the controls. The presence of pathogenic bacteria, as well as high bacterial counts, were associated with a higher number of symptoms. However, the predictive value of the bacteriological examination was low. Our study indicates that bacteriological examination of breast milk is justified only in patients with severe, acute symptoms and recurrences when betalactamase producing Staphylococcus aureus are suspected.