Nailfold capillary microscopy was carried out in patients with systemic sclerosis (58), systemic lupus erythematosus (SLE, 41), rheumatoid arthritis (RA, 27) and 27 normal subjects using a semiquantitative methodology. Mean enlarged capillary loop counts of greater than 2 were seen in 86% of the systemic sclerosis and 41.5% of the SLE but none of the RA patients or normals. Avascular changes were significantly more severe in systemic sclerosis. In systemic sclerosis the mean avascular score correlated with disease duration and tended to be higher in those with diffuse skin, pulmonary, cardiac and renal involvement. In SLE enlarged capillary loops were associated with Raynaud's phenomenon and less frequent renal involvement. Other morphological abnormalities observed were found to be entirely nonspecific. The technique employed was shown to be reproducible.