Methane (CH4) emissions from dairy cows are technically difficult and expensive to measure. Recently, some researchers have found correlations between the concentrations of specific fatty acids in milk fat and the CH4 emissions from cows that could obviate the need for direct measurement. In this research, data on individual cow CH4 emissions and concentration of caprylic acid (C8:0) and total C18 fatty acids in milk were collated from eight experiments involving 27 forage-based diets and 246 Holstein-Friesian dairy cows. Linear regressions between CH4 and both C8:0 and total C18 in milk were produced for published data and used to calculate 95% prediction regions for a new observation. The proportion of observed methane emissions from eight experiments that fell outside the 95% prediction region was 27.6% for the C8:0 model and 26.3% for the total C18 model. Neither model predicted CH4 emission well with Lin’s coefficient of concordance of less than 0.4 and the Nash–Sutcliffe efficiency coefficient of approximately zero for both the C8:0 and total C18 models. In addition, general linear model analysis showed significant differences between experiments in their intercepts (P < 0.001) and slopes (P < 0.001). It is concluded that the relationships tested cannot be used to accurately predict CH4 emissions when cows are fed a wide range of diets.