We present the results of investigation of peculiarities of the secular brightness fading for 18 short-period comets. Sekanina's results are supplemented by new data on apparitions of periodic comets after 1964. For each comet, changes of absolute magnitude per one year and per one orbital period together with their average values are determined. The statistically significant (factor of correlation is 0.81) influence of the 90-year cycle of the solar activity on secular variations of integrated magnitudes of short-period comets is found.