For individual phospholipid-coated microbubbles with a diameter smaller than 5 μm, highspeed camera recordings revealed threshold behavior in the onset of vibration. Previous studies showed a pressure-dependent attenuation for phospholipid-coated microbubbles. The aim of this study was to investigate if there is a relationship between microbubble threshold behavior and pressure-dependent attenuation. At a rising acoustic pressure from 5 to 200 kPa, the attenuation of BR14 increased linearly from 4.5 dB to 7 dB. In contrast to BR14, Levovist did not show pressure-dependent attenuation. For BR14 smaller than 3 μm diameter, attenuation increased from 4 to 12 dB. However, up to 50 kPa no increase of attenuation was observed. These observations can be explained by phospholipid-coated bubble threshold behavior. When the acoustic pressure increases above a threshold acoustic pressure, microbubbles smaller than 5 μm start to oscillate and contribute to the attenuation. Levovist is considered to be a free microbubble, for which threshold behavior is not expected. An imaging technique such as power modulation imaging could profit from the presence of an acoustic pressure threshold.