In a prospective open study, 24 patients with cancer pain receiving parenteral opioids were administered highly concentrated solutions of hydromorphone in order to assess the local tolerance to the subcutaneous infusion. Patients received a mean concentration of hydromorphone of 30 +/- 15 mg/mL at a mean rate of infusion of 0.3 +/- 0.25 mL/hr. Of 22 evaluable patients, 17 (77%) had a site duration of more than 7 days with their first subcutaneous site (mean concentration of 34 +/- 8 mg/mL of hydromorphone). Four of the five remaining patients (80%) had site duration of more than 7 days with their second subcutaneous site. Only one patient (5%) did not reach a 7-day duration after three consecutive site changes. Out of 7 subcutaneous sites that presented signs of toxicity before 7 days of infusion, the main problem was erythema in three cases, swelling in two cases, and bleeding in two cases. All sites improved spontaneously and did not require any medical treatment. No correlation was found between the concentration or rate of infusion of hydromorphone and the duration of the subcutaneous site. The duration of the site was significantly correlated with weight, tricep skinfold, subscapular skinfold, and age of the patients. Our findings suggest that hydromorphone can be safely administered in concentrations that are much higher than those commercially available. The availability of highly concentrated formulations of different opioids for parenteral use will make home management simpler and cheaper.