Differential loss to follow-up can substantially influence the evaluation o
f treatment effects on a dichotomous outcome of interest in longitudinal tr
ials. The use of transitional models incorporating loss to follow-up as an
additional category of response and the nature of the correlated responses
can provide a comprehensive view of a trial with unbalanced loss to follow-
up. Under the Markov assumption, transitional models estimate the probabili
ty of changing from one outcome to another outcome between follow-up visits
. Patterns of the response variable can be described by the estimated trans
ition probabilities. The effects of intervention and covariates on the outc
ome of interest can also be estimated using a conditional likelihood functi
on or a multinomial legit regression. Data from a randomized barrier method
study designed to compare the proportion of participants using barrier met
hods consistently in two counselling groups are used to illustrate the prop
osed model. Copyright (C) 2000 John Wiley & Sons, Ltd.