The Multicenter Study of Hydroxyurea in Sickle Cell Anemia (MSH), a double-blind randomized clinical trial, compared the frequency of acute vaso-occlusive (painful) crises during 2 yr of follow-up in 299 patients randomly assigned to hydroxyurea or placebo. Most patients had more than one crisis; all crises reported were included in the primary outcome analysis. A total of 7,229 follow-up medical contact reports were classified as crises/not crises by a Crisis Review Committee. Because of the time required to report, document, and classify contacts, interim analyses were prepared with incomplete data. If a stopping boundary were crossed, early termination could be advised only after assessing the potential impact of the incomplete data. In an extension of stochastic curtailment methods, simulation procedures were used to estimate the probability of detecting differences when group crisis rates projected to the end of the study were compared using a rank test. To account for medical contacts not yet reported and the future occurrence of crises, Poisson process models assuming no treatment effect on crisis rates were used for these simulations. The number of unclassified contacts that would be classified as crises was simulated as a binomial random variable. These methods may be useful for interim monitoring in other studies of recurrent events with ongoing event reporting and classification.