The acoustic effects of 1-, 2-, and 3-mm vents were investigated with in-the-ear, in-the-canal, and completely-in-the-canal hearing aid shells. Real-ear sound pressure level measures were obtained from unvented and vented shells with 12 adults. In general, with increasing vent size, a statistically significant (p < .05) increase in the amount of low-frequency reduction, an upward shift in vent cutoff frequencies, and an upward shift in vent-associated resonances occurred for all hearing aid shell styles. There was no significant change in the slope of the low-frequency reduction across all hearing aid shell styles (p > .05), albeit the frequency response curves were shifted upward in frequency with increasing vent diameters. Only with the in-the-ear and completely-in-the-canal hearing aid shells were statistically significant (p < .05) differences found with the magnitude of vent-associated resonance as a function of vent diameter, and these differences were not consistent across the different styles. These findings suggest that venting may be used effectively to tune low-frequency responses in custom in-the-ear hearing instruments.