We use a constraint-based language to specify periodic temporal patterns. The Constraint-based Pattern Specification Language (CAPSUL) is simple to use, but allows a wide variety of patterns to be expressed. CAPSUL solves problems such as (1) how to use calendar-based constraints to define repetition of a periodic event, (2) what temporal relations must exist between consecutive repeats of a pattern, and (3) how expressivity is limited if the same temporal relations must hold between each pair of intervals in the pattern. We implemented CAPSUL in a temporal-abstraction system called R?sum?, and used it in a graphical knowledge-acquisition tool to acquire domain-specific knowledge from experts about patterns to be found in large databases. We summarize the results of preliminary experiments using the pattern-specification and pattern-detection tools on data about patients who have cancer and have been seen at the University of Chicago bone-marrow-transplantation center.