This prospective study was conducted at Bamrasnaradura Hospital from November 11, 2002, until January 5, 2003, in order to describe the clinical manifestations and determine the aetiologies as well as to assess the short-term outcome of interstitial pneumonitis in HIV/AIDS patients. 59 patients with interstitial infiltrates on chest radiographs were included in the study. Tuberculosis (TB) was the most common diagnosis (44%), followed by Pneumocystis pneumonia (PCP) (25.4%), bacterial pneumonia (20.3%) and fungal pneumonia (10.2%). In TB, compared to other diagnoses, a mild cough (p = 0.031), pallor (p = 0.021), lymphadenopathy (p < 0.001), an absence of skin lesions (p = 0.003), a higher mean body temperature (p = 0.004) and an absence of dyspnoea on exertion (p = 0.042) were significant findings. In PCP, compared to other diagnoses, dyspnoea on exertion (p = 0.014), nonpurulent sputum production (p = 0.047), a higher mean respiratory rate (p < 0.001), and an absence of lymphadenopathy (p < 0.001) were significant factors. In bacterial pneumonia, compared to other diagnoses, production of purulent sputum (p = 0.014), haemoptysis (p = 0.006), skin lesions (p = 0.002) and severe cough (p = 0.040) were significantly associated factors. In fungal pneumonia, compared to other diagnoses, headache and papulonecrotic skin lesions were common findings, but no factor showed a significant association. After four weeks, 59.3% patients were alive and 13.6% had died. Among those alive, 88.6% had clinically improved. The cumulative survival after 28 days was highest among PCP patients, followed by bacterial pneumonia, TB and fungal pneumonia, but these differences were statistically not significant (p = 0.453).