We discuss the program, TWINAN90, which can perform several different types of analysis of twin data. TWINAN90 incorporates the ANOVA-based twin analyses from the TWINAN twin analysis program, and also includes maximum likelihood estimation of parameters from three path models. Another feature of TWINAN90 is the optional output of a pedigree file which can be read by the quantitative genetics package FISHER. The diagnostic features of the program make TWINAN90 useful also for preliminary analyses prior to the use of more sophisticated modeling procedures which are available in packages such as LISREL and FISHER. An annotated printout from TWINAN90 is presented to illustrate the statistical analyses performed in the program.