In a large kinship affected by myotonic dystrophy (MyD), 133 subjects, including 21 spouses, were examined independently by a neurologist and an ophthalmologist to determine the earliest identifying evidence of the disease. Assessment included tonometry, slit-lamp examination, facial measurements, and electromyography (EMG). Thirty-two subjects, all of whom had EMG characteristics of myotonia, were definitely affected. Low intraocular tensions were a more consistent finding than lens opacities; endocrine abnormalities were absent. Twenty-seven subjects had one to five features but lacked clinical or electrographic evidence of myotonia; they were designated as having a "partial syndrome." Signs most commonly found included weakness of the upper face, brachial hyporeflexia, low intraocular tensions, and the presence of sparse colored specks at the posterior poles of the lenses. All but 2 subjects (siblings) with the partial syndrome had a parent with either MyD or the partial syndrome. Linkage studies indicated that 7 subjects should not be carrying the gene, while others could be regarded as having a minor expression of MyD. In most persons with the partial syndrome, we could not predict whether typical MyD would develop later. Transmission of the disease may depend upon more than a single autosomal dominant gene, and nongenetic influences may also be important.