This paper presents diurnal variations of precipitation as observed using the Advanced Microwave Sounding Unit instruments, AMSU-A and AMSU-B, aboard the NOAA-15, NOAA-16, and NOAA-17 satellites. The method used for estimating precipitation was described by Chen and Staelin (IEEE Trans. Geosci. Remote Sensing, 41, 410-417, 2003) and Chen (Ph.D. thesis, MIT EECS Dept., 2004). This method relies primarily on opaque microwave channels in the 54-GHz oxygen and 183-GHz water vapor resonance bands. NOAA-15, NOAA-16, and NOAA-17 are polar-orbiting satellites, so AMSU obtains global observations about six times a day. The local equatorial crossing times of these satellites are about 7 AM/PM, 2 AM/PM, and 10 AM/PM, respectively. The nearly-even positioning of these satellites facilitates a study of the diurnal variation of precipitation. A space-time resolution of 10^°x10^° by 3 months seems to be appropriate for studying the diurnal variations of precipitation. Diurnal variations of precipitation frequency were computed for AMSU data from July 2002 to June 2003. For this purpose, precipitation events were defined as observations where the retrieved precipitation rate was greater than a specified threshold. For this paper, the thresholds used included 0.1, 1, and 2 mm/h. It was observed that in the western Pacific part of the ITCZ the local preferred time of precipitation frequency varied from about midnight to about 0500 as the threshold was increased from 0.1 to 2 mm/h, and similar trends were observed in other parts of the ITCZ. This suggests that future studies on the diurnal variations of precipitation might focus on specific types of precipitation or ranges of precipitation rate.