In this paper, we describe a distributed transactional workflow system named ICU/COWS, which supports multiple workflow types of large scale enterprises. The system aims to support the whole workflow for large scale enterprises effectively within a single workflow system and the system is designed to satisfy several design goals such as availability, scalability, and reliability. Transactional task and special tasks such as alternative task and compensating task are developed and utilized to achieve the design goals in task model level and the system is constructed with distributed transactional objects to achieve the design goals in distributed system environment. In this paper, structured ad hoc workflow is defined as a special type of ad hoc workflow that should be automated by workflow management system because many benefits can be obtained by automating it and connector facility is proposed as a means to support structured ad hoc workflow effectively. Some characteristics of a workflow system can be identified by monitoring the system behavior on different conditions like workloads or system configurations. An early version of the system has been implemented and the performance data of the system is illustrated.