Intra-uterine pressure recordings in normal and dysmenorrheic women during the menstrual period were analysed regarding amplitude, duration, frequency and peak area of contractions and the resting pressure. The activity of the myometrium showed large between subject variability and no significant difference was found, between the two groups in amplitude and duration of contractions or their frequency. However, there was a consistent difference in peak area at the onset of the menses (p less than 0.01) and there was also a significant decrease in peak area during the menses in the group with dysmenorrhea, with maximum uterine work being performed on the day when the pain was at its worst. The contribution of the pattern of contractility to the etiology of dysmenorrhea is discussed. Abnormal uterine activity as a cause of primary dysmenorrhea was first suggested in the 1930s and 1940s on the basis of intra-uterine pressure recordings (Moir, 1936; Woodbury et al., 1947). Since then, intra-uterine pressure has been measured in a number of studies using a micro-balloon (Lundström et al., 1976; Bygdeman et al., 1979). With the introduction of the highly sensitive micro-transducer catheter it has been possible to make accurate quantitative estimations (Akerlund et al., 1978; Ulmsten & Andersson, 1979) but, to our knowledge, no detailed analysis and quantification of intrauterine pressure recordings in primary dysmenorrhea has been reported. In this paper we publish the results of a study in which daily recordings obtained from women suffering from primary dysmenorrhea were compared with those from a group of women with very similar menstrual patterns but with minimal or no symptoms.