One hundred and fifty-one patients intolerant to zidovudine (AZT) received didanosine (ddI) to a maximum dose of 12.5 mg/kg/day. Patient response was assessed using changes in CD4+ lymphocyte subset count, HIV p24 antigen, weight, and quality of life. Seventy patients developed major opportunistic infections whilst on therapy; this was the first AIDS diagnosis in 17. Only minor changes in CD4+ lymphocyte subset count were observed in AIDS patients, although a more significant rise occurred in those with earlier stages of disease. Of those positive for p24 antigen at the commencement of the study 67% showed a positive response, and this was most likely in those with CD4+ lymphocyte subset counts above 100 mm3. A positive weight response was seen in 16% of patients. Most patients showed improvement in individual parameters and global score of quality of life. Adverse reactions possibly attributable to didanosine were common. The most common side-effect was diarrhoea, which resulted in cessation of therapy in 19 individuals. Peripheral neuropathy occurred in 12 patients and pancreatitis in six. Thirteen patients developed a raised serum amylase without abdominal pain. Seven patients developed glucose tolerance curves characteristic of diabetes but these were mild, did not require treatment and returned to normal on ceasing didanosine.