The safety of cefaclor advanced formulation (cefaclor AF) was evaluated in 3,272 patients participating in 11 controlled clinical trials in comparison with cefaclor (2,210 patients) for a variety of infectious illnesses. Daily doses of cefaclor AF ranged from 500 to 1500 mg, with a mean duration of treatment of 8.1 days (range 1-18 days). There were no significant differences between the cefaclor AF- and cefaclor-treated groups in the frequency of adverse events by body system for all events reported. The majority of adverse events related to therapy were mild and transient. Severe adverse events occurred in 2.1% of the cefaclor AF group and 2.7% of the cefaclor group. The most frequently reported adverse events for cefaclor AF were diarrhoea (3.4%), headache (3.2%), nausea (2.5%) and vaginal moniliasis (2.5% of females). Drug-related adverse events led to early discontinuations in 1.7% of cefaclor AF-treated patients and 1.6% of cefaclor-treated patients. Overall, there were few significant differences in the frequency of adverse reactions between older and younger patients. Notably, elderly patients reported significantly less diarrhoea and fewer hypersensitivity-type reactions. There were, however, more therapy discontinuations due to adverse effects in patients aged 65 years or older than in those less than 65 years of age. Many of the discontinuations were thought to be unrelated to therapy. Alterations in laboratory values in patients treated with cefaclor AF were similar to those seen with other beta-lactam antibiotics. The comprehensive data indicate that cefaclor AF is a safe therapeutic option for a variety of common bacterial infections.