Volume, fat and energy contents were measured in human milk samples collected over 2 24-h periods with either an electric or hand-operated breast pump. The energy content was significantly higher and the variance for this measurement was significantly lower in samples obtained by the electric pump. The type of pump used is an important consideration when the content of energy and other nutrients associated with the fat compartment in human milk is to be evaluated. This observation is especially pertinent to cross-study comparisons, eg studies of well- and malnourished populations.