The radial evolution of the power spectra of Alfv¿nic fluctuations observed within the trailing edge of high speed streams in the solar wind has been investigated using magnetic field measurements by Pioneer 10 and Pioneer 11 at heliocentric distances between 1 and 5 AU. In the analyzed frequency range (2.8¿10-4 to 8.3¿10-3 Hz) the spectral index of the computed spectra does not depend on frequency and is close to the value predicted for hydrodynamic turbulence. The radial gradient of the power density is independent of frequency and only slightly steeper than that expected for undamped propagation of Alfv¿nic fluctuations in a rarefaction region. The relation between the present results and those previously obtained by Helios 1 and 2 inside 1 AU is discussed.