We have evaluated the pharmacokinetics, reliability and patient tolerability of a newly developed injection pen for cartridged growth hormone (GH). The cartridge contains 25 IU GH in 2 ml solvents. The pen, which is basically a needle, syringe and vial in one piece, is operated by a turning movement and allows doses from 0.25-4 IU. Nine GH deficient patients were hospitalized twice for overnight bloodsampling following subcutaneous injections (at 8 p.m.) of GH: i.e. when using traditional syringe and vial and after 6 weeks of use of the pen. Serum GH antibodies were measured immediately prior to, and 3 and 6 months following pen treatment. GH containers were collected regularly from the patients for chemical analysis. A questionnaire was completed during and at the end of the study. The absorption rate and bioavailability of GH tended to be higher with syringe and vial (2 P = 0.07) but there were no differences in the profiles of IGF-I, insulin, glucagon or pertinent metabolic parameters following the 2 injection modes. No GH antibodies occurred during 6 months of pen treatment. The content of polymeric GH was lower in the cartridges (2 P less than 0.001). Seven of the patients reported less injection pain when using the injection pen, which they all strongly preferred and wished to continue using. We conclude that the GH injection pen is a reliable tool which seems to be more convenient for the patients.