BACKGROUND Succinylcholine-induced masseter muscle rigidity (MMR) is a potentially life-threatening complication of anesthesia and is closely correlated with the heterogeneous disorder malignant hyperthermia (MH) susceptibility. MMR also is identified with a variety of neuromuscular disorders, including the myotonias, that are associated with abnormal in vitro contracture test (IVCT) results. Recently, mutations in the adult skeletal muscle sodium channel alpha-subunit gene (SCN4A) have been shown to cause generalized nondystrophic myotonias, some of which are associated with mild nonspecific symptoms. The purpose of the current investigation was to begin to evaluate the molecular genetic relationship between known mutations in the SCN4A gene, MMR, and the results of the IVCT used to diagnose MH-susceptibility. METHODS A single extended pedigree of 16 individuals was ascertained through a proband who experienced MMR and whole-body rigidity after succinylcholine administration. Subsequently, four individuals were shown to have a mild form of myotonia on clinical and laboratory examination. IVCT was carried out according to standardized protocols. Mutations in the SCN4A gene were sought in exons 22 and 24 using single-strand conformational analyses. Variability in the SCN4A gene sequence was confirmed by direct DNA sequence analyses. RESULTS Four individuals with myotonia were shown to carry a guanine-to-cytosine mutation at nucleotide position 3917 of the reported SCN4A sequence. This DNA mutation was coinherited with MMR and an abnormal IVCT result in this family. Previous studies have demonstrated that the glycine1306-to-alanine substitution is associated with a mild clinical syndrome referred to as myotonia fluctuans. CONCLUSIONS The current report provides direct evidence that succinylcholine-induced MMR, whole-body rigidity, and an abnormal IVCT result are associated with a mutation in the SCN4A gene.