1. Body-weight, body height and skinfold measurements were taken in 419 adult males working in the UK offshore oil industry. Percentage body fat was estimated from skinfold thicknesses and the Quetelet index (weight:height) determined. 2. The prevalence of overweightness, assessed from the Quetelet index, in the age groups 20-29, 30-39 and 40-49 years was 31.6, 50.0 and 66.2% respectively. The Office of Population Census and Surveys (1981) showed that for age-matched groups, the incidence of overweightness in an onshore population was 26, 40 and 50% respectively. 3. The percentage body fat for each respective age group was greater than that reported for an age-matched onshore population. 4. The Quetelet index was significantly related to body fat (r 0.765, P less than 0.0001) and poorly correlated with height, thus this weight:height relation may be utilized in the assessment of overweightness in offshore personnel. 5. In conclusion it appears that the offshore population had a higher percentage body fat than their onshore peers and that the prevalence of overweightness was also greater.