Noctilucent cloud (NLC) observations in northwest Europe show a significant secular increase in the occurrence frequency between 1964 and 1994. Although model computations predicted a brightness increase due to increasing stratospheric methane concentrations and a consequent increase in the number of observed NLCs, earlier investigations of the histograms of the solar depression angles at the first and last NLC sightings did not reveal any noticeable brightness change. However, a reinvestigation of the histograms based on statistical model predictions, χ2 tests, and least squares fitting clearly indicates that the NLC brightness increased by a factor of the order of 5 between the periods 1964--1974 and 1985--1994. The brightness increase goes along with a significant lengthening of the NLC season. Considering recently published trends in the upper mesospheric water vapor mixing ratio and temperature strongly suggests that the brightness increase is caused by an increase of the water vapor concentration rather than a decrease in the temperature. For 1985--1994 the analysis also predicts at least a doubling of the NLC brightness between dusk and dawn, in good agreement with lidar observations.