The aim of this study was to determine whether the ultrastructure of the non-neoplastic myometrial portion (host myometrium) of fibromyomatous uteri is normal or abnormal when compared to that of fibromyomata and normal myometria. Myometrial samples from 23 normal and 54 fibromyomatous uteri were examined at the ultrastructural level using standard electron microscopy techniques. Ultrastructural abnormalities of certain cellular organelles were noted in myocytes of fibromyomata but not in those of normal or host myometria. The sarcolemmal dense bands of host myometrial myocytes were of significantly greater length than those of normal myometria, but not significantly different to those of fibromyomata. Consequently, the numbers of caveolae in host myometria and fibromyomata are conceivably decreased in comparison to normal myometria. Host myometria can be, therefore, considered to be structurally abnormal. The specific structural abnormality noted may affect calcium metabolism in these tissues by causing a decrease in the cellular calcium extrusion mechanism and thus raising intracellular calcium concentrations. Such an abnormality may provide an answer, in terms of contraction abnormalities, for the unexplained infertility that occurs in a small percentage of symptomatic myomatous patients.