This volume constitutes the refereed proceedings of the 12th International Workshop on Languages and Compilers for Parallel Computing, LCPC'99, held in La Jolla, CA, USA in August 1999. The 27 revised full papers and 14 posters presented have gone through two rounds of selection and reviewing. The volume offers topical sections on Java, low-level transformations, data distribution, high-level transformations, models, array analysis, language support, and compiler design and cost analysis.