Absolute thresholds and thresholds of pure tones masked by thermal noise were obtained for 3666 persons attending the 1948 San Diego County Fair. The masked hearing losses as well as the absolute hearing losses (1) increased with an individual's age, (2) were positively correlated with his awareness of hearing difficulty, and (3) were inversely related to his musical training. Masked losses, however, were less dependent upon one's sex and upon one's past noise environment than were absolute losses.When groups of individuals were equated as to their absolute losses on 880 c.p.s. and 3520 c.p.s. (the frequencies which were masked), that part of each group that had the lower absolute thresholds on 440 c.p.s. also had the lower masked thresholds.The signal to noise ratio (noise expressed as spectrum level) at masked threshold for those with no absolute loss was 20 db at 880 c.p.s. and 21 db at 3520 c.p.s. This is equivalent to saying that the noise bands which were effective in masking these frequencies were 100 and 128 cycles wide, respectively.