Irinotecan (CPT-11) is a promising camptothecin analogue with activity in solid tumors. In Phase I studies using short intravenous infusion schedules, the predominant drug toxicities have been gastrointestinal, such as diarrhea, and myelosuppression. In animal studies, prolonged infusion schedules followed by short drug-free intervals have resulted in preservation of antitumor activity and decreased drug toxicity. Therefore, we propose to examine a 96 hour infusion schedule of irinotecan alternating with 72 hour drug-free intervals in patients with solid tumors in order to determine the maximum tolerated dose of this regimen. The duration of these treatments will be escalated in our Phase I study until patients are receiving therapy for 2 out of every 3 weeks. Standard Phase I drug toxicity and tumor response information will be monitored and the pharmacokinetics of irinotecan and its active metabolite, SN-38 will also be examined. We will also attempt to monitor the intracellular molecular effects of SN-38 therapy in blood, bone marrow, and, whenever possible, tumor tissue.