N <- 20
myDf <- data.frame(x=rnorm(N, 0, 1),
                   y=rnorm(N, 0, 1),
                   w=runif(N, 0, 1))
myDf[c(1, 3, 5),  "y"] <- NA
myDf[c(2, 7, 14), "w"] <- NA
fitO <- lm(y ~ x, na.action=na.omit, weight=w, data=myDf)
length(residuals(fitO)) # -> 14

fitE <- lm(y ~ x, na.action=na.exclude, weight=w, data=myDf)
length(residuals(fitE)) # -> 20