Does femara (letrozole) cause weight gain?

Not typically. Estrogen in excess causes weight gain. If you block estrogen formation, it actually may prevent weight gain in certain cases. Low or overly blocked estrogen may actually cause weight gain (stimulates belly fat cells to try and make your own estrogen). Therefore estrogen in balance keeps you weight neutral.
Unclear. It's a common complaint, occuring in 1 out of 10 patients taking this drug but the data is unclear. Because it's a cancer drug, it is not compared to placebo so we can't say that it causes weight gain. If this happens to you, talk to your doctor. He/she might be able to lower the dose, switch to different med or advise on a diet/way to keep weight under control while taking this med.