Intervals of long-duration radial interplanetary magnetic fields (IMFs) were studied using the National Space Science Data Center OMNI 2 database between 1963 and 2003. Long-duration radial IMF events were detected in various regimes of solar wind. It was found that long-duration radial IMF events frequently occur around solar minimum and are more rare near solar maximum. The events tend to occur in the regions of decreasing solar wind speed through the solar cycle, and averaged decreasing rate of speed was approximately -2.6 km/s/h. The rate of decrease of the solar wind speed associated with the events does not clearly depend on the solar activity.