Serum triglyceride (TG) level is an important independent risk factor for coronary heart disease, with the lipoprotein lipase (LPL) enzyme playing the major role in regulating the catabolism of TG rich lipoproteins. The complete sequence analysis of the LPL gene was carried out on 19 individuals with extreme hypertriglyceridaemia (HTG; TG>14 mmol/l) with a total of 42 sequence variants being identified, a number of which are novel to this study. A total of eight patients were shown to have functional variants (p.D36N, p.R197H, p.N318S, p.V340I) that alter amino acids at 11 of the 16 LPL alleles. Multiplex ligation-dependent probe amplification (MLPA) analysis showed no exonic deletion or duplications in this population. Further analysis of the p.N318S (also called N291S) variant identified in the sequencing screen, in larger case and control populations, identified this mutation to be strongly associated with HTG. This study has produced a more comprehensive SNP map of LPL and its surrounding area and identified p.N318S as a major predisposing factor to HTG in the Northern Irish population.