Hereditary internal anal sphincter myopathy causing proctalgia fugax and constipation. A newly identified condition. A newly identified myopathy of the internal anal sphincter is described. In the affected family, at least one member from each of five generations had severe proctalgia fugax; onset was usually in the third to fifth decades of life. Three members of the family have been studied in detail. Each had severe pain intermittently during the day and hourly during the night. Constipation was an associated symptom, in particular difficulty with rectal evacuation. Clinically the internal anal sphincter was thickened and of decreased compliance. The maximum anal canal pressure was usually increased with marked ultraslow wave activity. Anal endosonography confirmed a grossly thickened internal anal sphincter. Two patients were treated by internal anal sphincter strip myectomy; one showed marked improvement and one was relieved of the constipation but had only slight improvement of the pain. The hypertrophied muscle in two of the patients showed unique myopathic changes, consisting of vacuolar changes with periodic acid-Schiff-positive polyglycosan bodies in the smooth muscle fibers and increased endomysial fibrosis. In vitro organ-bath studies showed insensitivity of the muscle to noradrenaline, isoprenaline, carbachol, dimethylpiperazinium, and electrical-field stimulation. Immunohistochemical studies for substance P, calcitonin gene-related peptide, galanin, neuropeptide Y, and vasoactive intestinal peptide showed staining in a similar distribution to that in control tissue. A specific autosomal-dominant inherited myopathy of the internal anal sphincter that causes anal pain and constipation has been identified and characterized. 