['To study the ultrastructure of the inner and outer myometrium, in the presence and absence of uterine adenomyosis.', 'Case control blinded comparison.', 'University departments.', 'Four premenopausal women with and six without uterine adenomyosis as the sole pathology.', 'Multiple samples were studied using transmission electron microscopy.', 'Ultrastructure feature of the myometrium.', 'In uteri with adenomyosis, the myocytes exhibited cellular hypertrophy. The cytoplasmic myofilaments were less abundant. Abundant intermediate filaments formed cytoplasmic aggregates. The nuclei had a smooth outline with a clear ground substance, prominent nucleoli and peripherally arranged nuclear chromatin. There was occasional infolding of the nuclear envelope with entrapment of cytoplasmic organelles. The sarcolemmal bands were significantly longer and there were fewer caveolae. The perinuclear cell organelles were more distinct. The rough endoplasmic reticulum and Golgi apparatus were more prominent, denoting active protein synthesis, consistent with the observed cellular hypertrophy. All features were more prominent at the junctional zone.'] Is uterine adenomyosis associated with ultrastructural features of altered contractility in the inner myometrium?