This study was designed to differentiate among possible causes of an increased incidence of myopia and related symptoms among submariners by making a longitudinal comparison of the visual functions of two groups of subjects, submariners and National Guardsmen. Refractive error, visual acuity, phorias, accommodation, and depth perception were measured. Submariners showed statistically significantly greater losses in visual acuity, accommodation, and depth perception over a 3.5-year period than did the guardsmen, but the differences were very small. Comparison with data in the literature gives evidence of large shifts in refractive error for young men, both submariners and guardsmen, born in recent years. it is suggested that whatever factors are responsible for the increase in myopia in the general population are operating even more effectively among submariners.