Two studies have been done to establish recommendations for dosage and dose adjustment in the treatment of deep vein thrombosis (DVT) with low molecular weight heparin (LMWH). In the first, 56 patients were randomized in a double blind study to be treated either with unfractionated heparin (UFH) or LMWH s.c. every 12 h. Initial doses were given according to age and sex, disregarding bodyweight, and the dose was then adjusted when the peak plasma heparin concentration fell outside the desired range of 0.5-0.8 anti-FXa U/ml. There were fewer dose adjustments in the LMWH group. The correlation between injected dose (U/kg bodyweight) and the heparin concentration was higher in the LMWH group (r = 0.59) than in the UFH group (r = 0.38). The results suggest that, in order to obtain the desired heparin concentration, the initial dose of LMWH should be about 100 U/kg bodyweight every 12 h. In the second, open study, this dosage plan was followed in 15 patients. The peak heparin concentration on Day 2 ranged from 0.40 to 0.75 anti-FXa U/ml and adjustment was only required in 3 patients. Day to day variation in peak heparin activity in the individual patient varied little (CV 11-22%), and there was no accumulation. The results indicate that plasma heparin concentration is more predictable using LMWH than UFH, and they point to definite advantages in the use of LMWH in a bodyweight adjusted dosage.