Several long-term studies of haemodialysis patients have shown improved serum lipid profile associated with treatment with low molecular weight heparin (LMWH) as compared with unfractionated heparin (UH). This has been attributed to the fact that LMWH produces a less marked acute lipolytic response than UH. However, the information on the differences in long-term effects on tissue releasable lipases is limited. Post-heparin plasma lipase activities were measured at 6, 24 and 48 h after pre-dialysis heparin injections in seven patients on chronic haemodialysis during treatment with UH; these measurements were then repeated 2 and 6 months after treatment was switched to LMWH. The curves plotted from the results can be assumed to reflect the interdialytic lipolytic potential. In the case of lipoprotein lipase this was unchanged 2 months after treatment was switched from UH to LMWH but increased by a mean of 47% after 6 months. In the case of hepatic lipase there was no change in the interdialytic lipolytic potential. Thus, there was a slow increase in tissue releasable lipoprotein lipase stores after treatment was switched from UH to LMWH, probably reflecting a smaller loss of lipoprotein lipase after each LMWH injection. Hepatic lipase, in contrast, was not affected by the type of anticoagulation.