The period changes of 86 M5 RR Lyrae stars have been investigated on a one-hundred-year time base .
The published observations have been supplemented by archival Asiago , Konkoly and Las Campanas photographic observations obtained between 1952 and 1993 .
About two thirds of the O - C diagrams could be fitted by a straight line or a parabola .
21 RR Lyrae stars have increasing , 18 decreasing and 16 constant period .
The mean rates of period change of these variables are : \langle \beta \rangle = \langle \dot { P } \rangle = -0.006 \pm 0.162 \mathrm { d { Myr } ^ { -1 } % } , \langle \alpha \rangle = \langle P ^ { -1 } \dot { P } \rangle = -0.021 \pm 0.308 \mathrm { % Myr } ^ { -1 } .
Ten RR Lyrae stars show fast period decrease with \dot { P } < -0.10 \mathrm { d { Myr } ^ { -1 } } .
At least some of these variables may be in the pre-zero-age horizontal-branch ( ZAHB ) evolutionary stage .
The variables on the long-period sequence of the period-amplitude diagram are brighter than the other RR Lyrae stars of M5 and are in an advanced evolutionary stage moving off from the HB redward .
More than one third of the M5 RR Lyrae stars investigated have irregular period change .
The irregular period behaviour is relatively more frequent among the RRc ( RR1 ) stars ( 50 per cent ) than among the RRab ( RR0 ) stars ( 34 per cent ) .
A strict relationship has been found between the irregular period change and the Blazhko effect of M5 RRab stars .
This fact indicates a common origin for these phenomena .
It is remarkable that , if the RRab stars showing Blazhko effect are omitted from the sample , the mean rates of the period change have small positive values ( \langle \beta \rangle = 0.012 \pm 0.147 \mathrm { d { Myr } ^ { -1 } } , \langle \alpha \rangle = 0.013 \pm 0.279 \mathrm { Myr } ^ { -1 } ) , in excellent agreement with HB evolutionary model predictions .