In the analysis of medical data, one often encounters data that are observed sequentially over time. For example in AIDS studies, let $X_1$ denote the time of infection, $X_2$ the time when antibodies occur and $X_3$ the time when AIDS is diagnosed for the first time. Typically, the variables of interest are the lifetimes $U_1 = X_2 - X_1$ and $U_2 = X_3 - X_2$, which may be dependent. While in applications, $U_1$ is often truncated from the right, $U_2$ may be censored due to time limitations. It is the aim of this paper to statistically analyse the joint distribution of the pair $(U_1,U_2)$.