The intention-to-treat (ITT) approach to randomized controlled trials analyzes data on the basis of treatment assignment, not treatment receipt. Alternative approaches make comparisons according to the treatment received at the end of the trial (as-treated analysis) or using only subjects who did not deviate from the assigned treatment (adherers-only analysis). Using a sensitivity analysis on data for a hypothetical trial, we compare these different analytical approaches in the context of two common protocol deviations: loss to follow-up and switching across treatments. In each case, two rates of deviation are considered: 10% and 30%. The analysis shows that biased estimates of effect may occur when deviation is nonrandom, when a large percentage of participants switch treatments or are lost to follow-up, and when the method of estimating missing values accounts inadequately for the process causing loss to follow-up. In general, ITT analysis attenuates between-group effects. Trialists should use sensitivity analyses on their data and should compare the characteristics of participants who do and those who do not deviate from the trial protocol. The ITT approach is not a remedy for unsound design, and imputation of missing values is not a substitute for complete, good quality data.