In order to evaluate the urinary excretion of cobalt, a study was carried out in two plants producing diamond segments and sintered wires for stone cutting. In both plants, the highest Co exposures were found during mixing and granulation of Co powders, generally performed 1-2 times per week for 1-2 h in the first plant and 3-4 h in the other. Environmental hygiene conditions were good in the first plant and poor in the other with environmental Co concentrations around the ACGIH TLV (50 micrograms/m3) in the first and many times higher (up to 8000 micrograms/m3) in the second. In two workers employed in the above mentioned jobs. Cobalt in urine (CoU) was determined in samples collected during the first day, before shift and about 2, 4, 7 and 10 h after the beginning of exposure; samples were also collected on the 2nd, 3rd and 4th day, before and at the end of the shift. The study showed that Co in urine rapidly increased in the hours following the cessation of exposure, with a peak of elimination about 2-4 h after exposure, and a subsequent decrease (more rapid for the first 24 h) in the following days. Such a pattern was apparently independent of the degree of exposure, since it was detected in both plants and confirmed also in an industrial hygienist with lower Co exposure. The data indicate that for a correct evaluation of Co exposure through CoU determination, variables such as type and characteristics of the job and most importantly, the times at which biological samples are collected should be carefully considered.