Observational studies indicate that stratospheric water vapor has been increasing by 1% per year on average over the past few decades. The only well established mechanism for this increase is the increasing concentration of atmospheric methane and its oxidation in the stratosphere. However, this effect accounts for no more than half of the observed trend in water vapor. Here we use a middle atmosphere general circulation model (GCM) and the observed record of past El Nino/Southern Oscillation (ENSO) activity to test whether observed increases in water vapor can be explained by changes in ENSO activity. In our model, El Nino conditions typically result in about a 10% increase in the concentration of water vapor entering the stratosphere. The observed record also shows an increase of almost one extra year of El Nino conditions per decade over the past half century. The model and observational results taken together therefore imply a positive trend in water vapor of around 0.1% per year and ENSO effects appear to explain no more than about one tenth of the long term trend. We also find variability in the water vapor entry values related to the Quasi-Biennial Oscillation (QBO) in our model. As ENSO and the QBO are variable in nature, this helps to explain some of the intermittency in the observed trend of increasing water vapor, particularly during periods such as the first part of the 1990s when ENSO was relatively active and water vapor increased rapidly despite a reduction in the rate of methane increase.