Twenty patients with collagen diseases complicated with Pneumocystis pneumonia (PCP) were retrospectively examined in reference to the criteria for its protective therapy provided by the Ministry of Health Labor and Welfare. The breakdown of 20 patients was rheumatoid arthritis (RA) in 5 cases, systemic lupus erythematosus (SLE) in 5, dermatomyositis (DM) in 2, systemic scleroderma (SSc) in 1, mixed connective tissue disease (MCTD) in 1, Sjögren syndrome (SjS) in 1, polyarteritis nodosa (PN) in 3, rapidly progressive glomerulonephritis (RPGN) in 1, Schönlein-Henoch purpura in 1. Patients having interstitial pneumonia (IP) or renal dysfunction before acquiring PCP showed poor prognosis. High level of beta-D glucan was observed in all patients, and elevated levels of LDH and KL-6 were also characteristic of PCP. For the treatment of their own collagen diseases, high dose steroids had been given in 11 patients (55%), and immunosuppressive agents in 12 (60%), resulting in severe suppression of immune function in these patients. They were treated with Sulfamethoxazole/trimethoprim (ST) after Pneumocystis infection, however, 10 patients died and 8 of them died of respiratory failure in spite of high dose steroids. Nine patients fulfilled the criteria for PCP protective therapy provided by Ministry of Health Labor and Welfare, and 7 of them died of respiratory failure. The frequency of PCP remarkably decreased in our hospital after we had started the protective therapy with ST using the criteria, suggesting that it is effective for the protection of PCP. However, some patients who do not fulfill the criteria may acquire severe PCP.