Patients with moderate to severe cancer pain and insufficient pain relief from nonopioid analgesics were treated with slow-release tramadol for initial dose finding and as a long-term treatment. Immediate-release tramadol was provided for the treatment of breakthrough pain and a standard nonopioid analgesic (1000 mg naproxen daily) was given as suggested for step 2 of the WHO analgesic ladder. Ninety of 146 patients (62%) completed the 6-week trial period. Drop-outs were due to adverse events (20%), inadequate pain relief (9%), or both (2.5%), death due to the underlying disease (4%), low patient compliance (2%) or other reasons. Average and maximal pain intensity decreased from day 1 to day 4. The number of patients with good and complete pain relief increased from 43% after week 1 to 71% after week 6 with maximum daily doses of tramadol up to 650 mg. However, 70% of the patients still needed less than 400 mg tramadol per day in week 6. Most patients (86%) experienced adverse events during the study period. Some common side effects of opioids, such as fatigue, dizziness, and constipation, decreased in frequency over the 6 weeks. The frequency of other adverse events such as nausea, vomiting and sweating did not change. Slow-release tramadol provided fast and efficient pain relief in almost two-thirds of patients both during initial dose finding and during long-term treatment, improving treatment options in step 2 of the WHO analgesic ladder.