The term "EMG disease" is used by some to describe the unexpected finding of diffusely increased insertional activity on needle electromyography in an otherwise asymptomatic person. The cause is unknown, but it has been hypothesized that these patients actually have a subclinical myotonic disorder. We describe 2 patients with diffusely increased insertional activity on electromyography who had mutations of the CLCN1 gene associated with myotonia congenita. Neither patient had symptoms or reproducible signs of this disorder. We propose that asymptomatic patients with CLCN1 mutations may at least partially account for the EMG disease phenotype.