HLA typing was performed on 267 Welsh coalworkers with pneumoconiosis (96 cases of simple pneumoconiosis, 115 cases of progressive massive fibrosis and 56 cases of Caplan's Syndrome) and 134 coalworkers with no abnormality. The presence or absence of rheumatoid factor was also determined. The results fail to confirm a previously reported increase in HLA-A1 and B18 in coalworkers with no pneumoconiosis. When correction was made for the number of antigens typed (i) HLA-Bw21 was significantly increased from 1.1% in the total group with pneumoconiosis to 8.2% in coalworkers with no abnormality (P corrected less than 0.032); (ii) HLA-Bw45 was increased in Caplan's Syndrome (10.7%) and Caplan's Syndrome patients with rheumatoid factor (16.1%) when compared to a non-occupationally exposed control group (0.8%) (P corrected = 0.019 and 0.0064 respectively). These results were not significant when comparisons were made with the coalworker group with no abnormality. The apparent higher frequency of Bw45 in Welsh coalworkers is discussed.