Perennial allergic conjunctivitis (PAC) is a perennial form of seasonal allergic (hay fever) conjunctivitis (SAC). Fourteen subjects with PAC were compared with 25 subjects with SAC. All patients met the same criteria for the diagnosis of a Type 1 ocular allergy. In a study of ophthalmic morbidity at an inner city health centre during the summer months the 3-month period prevalence of PAC was 3.5:10,000. In PAC symptoms are present throughout the year although seasonal exacerbations occurred in 79 per cent. Many similarities were found with SAC. PAC showed the same age range, length of history and symptoms as SAC although SAC was more severe. In both, conjunctival cytology showed eosinophils (PAC 43 per cent; SAC 25 per cent) and total IgE levels were raised in the tears (PAC 100 per cent; SAC 96 per cent) and serum (PAC 79 per cent; SAC 78 per cent). However, PAC differed from SAC in several respects: a history of exacerbation on exposure to house dust (PAC 42 per cent; SAC none) and an association with perennial rhinitis (PAC 75 per cent; SAC 12 per cent) were more common in PAC. The largest response on skin prick testing to a panel of antigens was to house dust mite (Dermatophagoides pteronyssinus) and this was more frequent in PAC (PAC 71 per cent; SAC 4 per cent). Dust mite antigen specific IgE was present in the tears of 78 per cent of cases of PAC, in higher levels than in serum indicating local production of IgE, whereas in SAC there were no detectable levels in any of the tears tested.(ABSTRACT TRUNCATED AT 250 WORDS)