In a field study of occupational dermatoses in a prefabrication construction factory, 272 workers were interviewed, examined and patch tested to chromate, cobalt, nickel, rubber mixes, epoxy resin, melamine formaldehyde and conplasts. The prevalence of occupational dermatitis was 14% (38/272); 57% (22/38) were irritant dermatitis from cement; 39.5% (15/38) were allergic contact dermatitis from cement (2 with concomitant rubber glove allergy); and 2.5% (1/38) were allergic to rubber chemicals in gloves. The overall prevalence of chromate sensitivity was 8.5% (23/272), with the highest rate from the concreting bays of the factory. The rate was unrelated to the duration of workers' engagement in construction work. 34.8% (8/23) had asymptomatic chromate allergy. The prevalence of cobalt reactions was 17.4% (4/23) and all were associated with chromate allergy.