Aims:　It　was　the　aim　of　this　study　to　characterize　the　features　of　acute　and　chronic　pancreatic　pseudocysts　(PPs)　and　to　identify　the　factors　predictive　of　the　need　for　invasive　treatment.　Methods:　Thirty-six　patients　with　PPs　treated　at　Nippon　Medical　School　between　January　1995　and　December　2004　were　studied　retrospectively.　The　cases　were　divided　into　4　groups　based　on　4　features:　association　with　acute　pancreatitis,　association　with　chronic　pancreatitis,　spontaneous　resolution,　and　persistent　symptoms　requiring　therapeutic　intervention.　Group　1　included　9　patients　with　acute　PPs　which　resolved　spontaneously.　Group　2　included　9　patients　with　acute　PPs　with　persistent　symptoms　or　associated　complications　requiring　interventional　treatment.　Group　3　included　9　patients　with　chronic　PPs　which　resolved　spontaneously,　and　group　4　included　9　patients　with　chronic　PPs　with　persistent　symptoms　or　associated　complications　requiring　interventional　treatment.　Results:　Among　the　36　patients,　13　were　women　and　23　were　men.　The　etiologies　were　pancreatitis　due　to　alcoholism　in　18　cases　(50.0%),　biliary　tract　disease　in　8　cases　(22.2%)　and　other　conditions　in　10　cases　(27.8%).　The　average　duration　of　follow-up　was　24.2　+/-　18.5　months.　The　patients　in　group　1　were　significantly　older　than　those　in　group　2　(67.6　+/-　16.1　vs.　40.6　+/-　14.1　years;　p　=　0.011).　The　mean　size　of　the　PPs　was　significantly　larger　in　groups　1　and　4　than　in　group　3　(p　＜0.05)　and　significantly　larger　in　group　2　than　in　group　4　(p　＜0.05).　There　were　no　significant　differences　between　groups　1　and　2　in　the　size　of　the　PPs　or　in　the　Ranson　score　of　previous　pancreatitis.　The　increase　in　size　of　the　PPs　during　follow-up　in　each　of　the　spontaneously　resolved　groups　(groups　1　and　3)　differed　significantly　from　that　in　each　of　the　interventional　treatment　groups　(groups　2　and　4;　p　＜0.05).　The　main　cause　of　the　acute　pancreatitis　in　group　1　was　biliary　tract　disease,　while　that　in　group　2　was　alcoholism　(significantly　different,　p　＜0.05).　The　number　of　patients　with　symptoms　related　to　pseudocysts　at　the　time　of　diagnosis　was　significantly　higher　in　group　1　than　in　group　3.　Conclusions:　Growth　of　the　PPs　during　follow-up　is　the　strongest　predictor　of　the　need　for　invasive　treatment　in　both　acute　and　chronic　cases.　Among　acute　PPs,　the　size　of　the　pseudocyst　is　not　in　itself　a　predictor　of　invasive　treatment.　Invasive　treatment　may　pose　higher　risks　for　pseudocysts　with　an　etiology　of　alcoholic　acute　pancreatitis.　However,　the　size　of　the　pseudocyst　may　be　a　more　important　prognostic　factor　than　an　etiology　of　pancreatitis.　Copyright　(c)　2008　S.　Karger　AG,　Basel　and　IAP.