Positive patch tests to sodium metabisulfite (SMB) are frequent. Standard series patch testing to SMB in 1751 patients showed 71 reactions interpreted as positive and allergic. 33 (46.5%) reactions were originally reported as relevant and 38 (53.5%) were of unexplained relevance depending on the presence or absence of identifiable sources responsible for the presenting dermatitis. A breakdown of these findings is presented. An additional detailed study of the sources of SMB in the environment and a retrospective analysis of these results have been undertaken to identify further, possibly overlooked sources of SMB exposure based on the occupational and recreational history. Most of the positive reactions in the relevant group were attributed to the use of Trimovate cream (63%). 5 patients (13%) with positive reactions in the unexplained relevance group were potentially exposed to SMB in local anaesthetic solutions while at work. 3 patients in the unexplained relevance group (7.8%) and 4 (12.1%) in the relevant group had potential for occupational exposure to SMB as bakers or caterers. Overall, occupational exposure was considered as a possible source of sensitization in 10 (26.3%) patients in the unexplained relevance group. We propose that sensitization to SMB from parenteral solutions and occupational exposure from food handling may account for some of the otherwise unexplained positive patch test reactions. A detailed occupational history should be therefore be sought in otherwise unexplained positive reactions to SMB. We also propose that it is worthwhile including SMB in our standard series in the UK.