A double-blind, controlled study is presented on the effect of low-dosage heparin on acute calcaneal peritendinitis crepitans in young men. Treatment consisted in exemption from physical work and subcutaneous injection of heparin. 5000 I.U., or placebo, once a day for 5 consecutive weekdays. During the first week the mean total symptom score showed a gradual fall to 32 and 34% of the baseline values in the heparin (n = 10) and the placebo group (n = 10), respectively. On day 15 the value was unchanged in the heparin group, whereas it had increased again to 45% in the placebo group. However, the symptoms and signs recorded never differed significantly between the two groups. No adverse reactions were observed. Consequently, further studies are needed to show whether a treatment of acute peritendinitis can be obtained with heparin in a dosage with less risk of bleeding than that involved in the previously reported regimen.