Breast pain (mastalgia) is a common condition (usually classified as cyclical or non-cyclical) the characteristics of which have never been studied using a standardized pain instrument. We have modified the short form of the McGill Pain Questionnaire (SF-MPQ) for the measurement of mastalgia, and have administered it to 271 women with breast pain and without breast cancer. The mean pain-rating index (sum of 15 descriptors of SF-MPQ) was similar between cyclical and non-cyclical pain, and was 12.0 (of 45) for the entire group. When compared to similar studies of pain at other sites, this falls in the same range as chronic cancer pain, and just below the pain of rheumatoid arthritis. Mean %VAS (visual analog scale) was 45.12 and mean %PPI (present pain index) was 39.9. Most women described their pain as 'heavy, aching and tender,' and these descriptors were given significantly higher ratings by women with cyclical pain. In women with non-cyclical mastalgia, the overall pain severity was related to the size of the painful area, and the steadiness of the pain, and the affective components were more prominent than in women with cyclical mastalgia. Thus, cyclical and non-cyclical mastalgia show some differences in their characteristics with substantial overlap. The total breast pain score was most efficiently estimated by a combination of the VAS, the PPI, and the quality of life questions (R2 = 0.96). Studies of breast pain should include both groups to better understand and characterize these differences, particularly with regard to a possible connection with breast cancer risk.