Methods for the analysis of the joint transmission of two phenotypes are described and used to determine the extent to which lipoprotein concentrations share a common genetic and/or environmental background. Analysis of data on 160 Caucasian nuclear families revealed that the observed phenotypic association between high-density cholesterol (HDL) and low-density cholesterol (LDL) could be accounted for in terms of common family environmental effects alone (estimated genetic correlation, ρG = −0·132±0·136; estimated residual environmental correlation, ρR = 0·065±0·230). The association between HDL and very-low-density cholesterol (VLDL) could not be accounted for in terms of family environmental effects alone. For HDL and VLDL the residual environmental correlation was significant while the genetic correlation was not (ρG = −0·111±0·214, ρR = −0·421±0·172). The correlation between LDL and VLDL also could not be accounted for in terms of common family environmental effects alone, although here a genetic relationship appears to be the important factor (ρG = 0·330±0·192, ρR = 0·010±0·217).